简洁代码的艺术The Art of Clean Code

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简洁代码的艺术 The Art of Clean Code

“我的代码能跑了”！

不，这还不够！如果你想走得快，如果你想快速完成，如果你想让你的代码易于编写，那么就让它易于阅读。

“My code works”!

Nope, that’s not enough! If you want to go fast, if you want to get done quickly, if you want your code to be easy to write, make it easy to read.

下面就让我们来看一下简洁代码的一些复杂之处。

So let’s see a few intricacies（错综复杂的） of clean code.

变量名称：

📝变量名应该说明意图、存在的原因、作用和使用方法。

例如：

✅studentScoreCardInfo

❎scoreInfo

📝避免混淆名称（即使用已经暗示某些东西的变量名称）并选择清晰的名称（即说出你的意思和你说的意思）。

📝用好区分，避免在末尾使用数字。

✅productInfo,productDetails

❎list1,list2

📝使用可发音的名字。

✅dateOfBirthInYearMonths

❎dobyymm

📝使用可以搜索到的名字。

✅maxScore

❎m, i, j

📝不要在名称中编码类型（记住类型/容器可以改变）

❎costFloat（将来成本也可以是整数）

📝类名应该是名词，函数名应该有动词。例如班级名称：Student、Employee 等函数名称：deleteStudentId、updateStudentName。

📝始终如一地使用名字。例如，对于学生，不要同时使用Score和Marks；❎ 例如 maxScore 和 minMarks。

Variable Names:

📝Variable names should tell the intent, why it exists, what it does and how to use it. Eg.✅studentScoreCardInfo ❎scoreInfo

📝Avoid confusing names(i.e using variable names that already imply something) and choose clear names (i.e say what you mean and mean what you say).

📝Use good distinctions, avoid using numbers at the end. ✅productInfo,productDetails ❎list1,list2

📝Use pronounceable names.✅dateOfBirthInYearMonths ❎dobyymm

📝Use names that could be searched.✅maxScore ❎m, i,j

📝Don’t encode types in names(remember types/containers can change) ❎ costFloat(in future cost could be an integer as well)

📝Class names should be nouns and function names should have verbs. Eg Class Names: Student, Employee, etc Function Names: deleteStudentId, updateStudentName.

📝Use names consistently. Eg for a student, don’t use scores and marks both;❎ eg maxScore along with minMarks.

函数：

📝编写小的函数。

📝在你的函数中只做一件事。

📝遵循DRY（不重复）原则。

📝优先使用异常而不是错误代码。异常在调试（堆栈跟踪）方面更容易处理。

Functions:

📝Write functions that are small.

📝Do only one thing in your function.

📝Follow the DRY (Do Not Repeat) principle.

📝Prefer exceptions over error codes. Exceptions are easier to deal with in terms of debugging (stack trace).

注释：

📝你不应该注释什么：

❎真相只能在一个地方找到：代码。

不要写注释来解释代码。

注释不能弥补糟糕的代码。

❎不要写混淆的注释（😂你需要注释来解释注释的地方）。

❎不要在注释中提供太多细节（注释应该是总结概述性的）。

❎私有 API，所有函数的注释？NOOOOOO！

📝什么时候应该使用注释：

✅对于公共 API。

✅揭示代码背后的意图。

✅解释决策背后的理由和所选方法的理由。

✅强调某事或提供警告。

✅ 有时会解释“为什么”代码有效。（“HOW”是代码应该解释的东西）。

✅难以理解的代码“可以”使用注释。

✅注释方法/函数抛出的异常。

Comments:

📝What should you NOT with comments:

❎Truth can only be found in one place: the code. Don’t write comments to explain the code. Comments do not make up for bad code.

❎Don’t write confusing comments(😂where you would need comments to explain the comment).

❎Don’t provide too many details in comments.

❎Private APIs, comments for all functions?NOOOOOO!

📝When should you use comments:

✅For public APIs.

✅To reveal the intent behind the code.

✅To explain the rationale behind decisions made and justification for the approach chosen.

✅To emphasize something or provide caution.

✅ Sometimes to explain “WHY” the code works. (“HOW” is something code should explain).

✅Difficult to understand code “MAY” use comments.

✅Comment about the exceptions thrown by the methods/functions.

格式化

📝格式化有助于与您的开发人员更好地沟通。

📝代码中相似的功能/概念应该放在一起

📝最重要的功能排在第一位。

📝代码的水平对齐方式应该使开发人员不需要水平滚动条。

Formatting

📝Formatting helps in better communication with your fellow developers.

📝Similar functions/concepts within code should be grouped together

📝Most important functions come first.

📝Horizontal alignment of code should be in such a way that the horizontal scrollbar is not needed by the developer.

总结

“干净的代码”是指软件开发项目中代码质量的术语。它是易于阅读、理解和维护的代码，并且遵循软件工艺的最佳实践和标准。

"Clean code" is a term that refers to the quality of code in a software development project. It is code that is simple to read, understand, and maintain and adheres to best practices and standards of software craftsmanship.

有几个有助于干净代码的基本原则。其中包括：

可读性：干净的代码易于阅读和理解。它为变量、函数和类使用描述性和有意义的名称，并遵循一致的风格和格式。

简单性：干净的代码很简单，避免了不必要的复杂性。它采用简单的算法和数据结构，同时避免重复和冗余代码。

可维护性：干净的代码易于维护和修改。它是模块化的，具有定义明确的接口和清晰的关注点分离。它也有详细的文档记录，以便其他开发人员可以轻松理解它的工作原理。

可重用性：干净的代码旨在可重用，以便可以轻松地将其合并到其他项目中或用作新开发的起点。它结构良好，并遵循代码组织方面的最佳实践，例如设计模式和软件架构。

可测试性：干净的代码易于测试，因此开发人员可以确保它按照他们想要的方式工作，并更新和更改它，而不必担心破坏代码库的其他部分。它的编写方式使编写自动化测试变得简单，并遵循最佳测试和调试实践。

编写干净的代码有几个优点。它使开发人员更容易理解和修改代码，使他们能够更高效地工作。它还通过确保代码结构良好并遵循最佳实践来帮助防止错误和错误的发生。此外，干净的代码使与其他开发人员的协作变得更加容易，因为它易于理解和使用。

最后，干净的代码是软件开发的一个重要方面。它是易于阅读、理解和维护的代码，并且遵循编写代码的最佳实践和标准。通过编写干净的代码，开发人员可以更高效地工作，避免错误和错误，并改善与他人的协作。

There are several fundamental principles that contribute to clean code. Among these are:

Readability: Clean code is simple to read and understand. It employs descriptive and meaningful names for variables, functions, and classes and adheres to a consistent style and format.

Simplicity: clean code is simple and avoids unnecessary complexity. It employs simple algorithms and data structures while avoiding duplication and redundant code.

Maintainability: Clean code is simple to maintain and modify. It is modular, with well-defined interfaces and a clear separation of concerns. It is also well documented so that other developers can easily understand how it works.

Reusability: Clean code is intended to be reusable so that it can be easily incorporated into other projects or used as a starting point for new development. It is well-structured and adheres to best practices in code organization, such as design patterns and software architectures.

Testability: Clean code is easy to test, so developers can make sure it works the way they want it to and update and change it without worrying about breaking other parts of the codebase. It is written in a way that makes writing automated tests simple and adheres to the best testing and debugging practices.

There are several advantages to writing clean code. It makes it easier for developers to understand and modify code, allowing them to work more efficiently. It also helps keep errors and bugs from happening by making sure the code is well-structured and follows best practices. Furthermore, clean code makes collaborating with other developers easier because it is easy to understand and work with.

Finally, clean code is an essential aspect of software development. It is code that is simple to read, understand, and maintain and adheres to best practices and standards for writing code. By writing clean code, developers can work more efficiently, avoid errors and bugs, and improve collaboration with others.

代码实例

参考：https://github.com/ryanmcdermott/clean-code-javascript

Variables

Use meaningful and pronounceable variable names

Bad:

const yyyymmdstr = moment().format("YYYY/MM/DD");

Good:

const currentDate = moment().format("YYYY/MM/DD");

Use the same vocabulary for the same type of variable

Bad:

getUserInfo();
getClientData();
getCustomerRecord();

Good:

getUser();

Use searchable names

We will read more code than we will ever write. It's important that the code we do write is readable and searchable. By not naming variables that end up being meaningful for understanding our program, we hurt our readers. Make your names searchable. Tools like buddy.js and ESLint can help identify unnamed constants.

Bad:

// What the heck is 86400000 for?
setTimeout(blastOff, 86400000);

Good:

// Declare them as capitalized named constants.
const MILLISECONDS_PER_DAY = 60 * 60 * 24 * 1000; //86400000;

setTimeout(blastOff, MILLISECONDS_PER_DAY);

Use explanatory variables

Bad:

const address = "One Infinite Loop, Cupertino 95014";
const cityZipCodeRegex = /^[^,\\\\]+[,\\\\\\s]+(.+?)\\s*(\\d5)?$/;
saveCityZipCode(
  address.match(cityZipCodeRegex)[1],
  address.match(cityZipCodeRegex)[2]
);

Good:

const address = "One Infinite Loop, Cupertino 95014";
const cityZipCodeRegex = /^[^,\\\\]+[,\\\\\\s]+(.+?)\\s*(\\d5)?$/;
const [_, city, zipCode] = address.match(cityZipCodeRegex) || [];
saveCityZipCode(city, zipCode);

Avoid Mental Mapping

Explicit is better than implicit.

Bad:

const locations = ["Austin", "New York", "San Francisco"];
locations.forEach(l => 
  doStuff();
  doSomeOtherStuff();
  // ...
  // ...
  // ...
  // Wait, what is `l` for again?
  dispatch(l);
);

Good:

const locations = ["Austin", "New York", "San Francisco"];
locations.forEach(location => 
  doStuff();
  doSomeOtherStuff();
  // ...
  // ...
  // ...
  dispatch(location);
);

Don't add unneeded context

If your class/object name tells you something, don't repeat that in your variable name.

Bad:

const Car = 
  carMake: "Honda",
  carModel: "Accord",
  carColor: "Blue"
;

function paintCar(car, color) 
  car.carColor = color;

Good:

const Car = 
  make: "Honda",
  model: "Accord",
  color: "Blue"
;

function paintCar(car, color) 
  car.color = color;

Use default parameters instead of short circuiting or conditionals

Default parameters are often cleaner than short circuiting. Be aware that if you use them, your function will only provide default values for undefined arguments. Other "falsy" values such as '', "", false, null, 0, and NaN, will not be replaced by a default value.

Bad:

function createMicrobrewery(name) 
  const breweryName = name || "Hipster Brew Co.";
  // ...

Good:

function createMicrobrewery(name = "Hipster Brew Co.") 
  // ...

Functions

Function arguments (2 or fewer ideally)

Limiting the amount of function parameters is incredibly important because it makes testing your function easier. Having more than three leads to a combinatorial explosion where you have to test tons of different cases with each separate argument.

One or two arguments is the ideal case, and three should be avoided if possible. Anything more than that should be consolidated. Usually, if you have more than two arguments then your function is trying to do too much. In cases where it's not, most of the time a higher-level object will suffice as an argument.

Since JavaScript allows you to make objects on the fly, without a lot of class boilerplate, you can use an object if you are finding yourself needing a lot of arguments.

To make it obvious what properties the function expects, you can use the ES2015/ES6 destructuring syntax. This has a few advantages:

  1. When someone looks at the function signature, it's immediately clear what properties are being used.

  2. It can be used to simulate named parameters.

  3. Destructuring also clones the specified primitive values of the argument object passed into the function. This can help prevent side effects. Note: objects and arrays that are destructured from the argument object are NOT cloned.

  4. Linters can warn you about unused properties, which would be impossible without destructuring.

Bad:

function createMenu(title, body, buttonText, cancellable) 
  // ...


createMenu("Foo", "Bar", "Baz", true);

Good:

function createMenu( title, body, buttonText, cancellable ) 
  // ...


createMenu(
  title: "Foo",
  body: "Bar",
  buttonText: "Baz",
  cancellable: true
);

Functions should do one thing

This is by far the most important rule in software engineering. When functions do more than one thing, they are harder to compose, test, and reason about. When you can isolate a function to just one action, it can be refactored easily and your code will read much cleaner. If you take nothing else away from this guide other than this, you'll be ahead of many developers.

Bad:

function emailClients(clients) 
  clients.forEach(client => 
    const clientRecord = database.lookup(client);
    if (clientRecord.isActive()) 
      email(client);
    
  );

Good:

function emailActiveClients(clients) 
  clients.filter(isActiveClient).forEach(email);


function isActiveClient(client) 
  const clientRecord = database.lookup(client);
  return clientRecord.isActive();

Function names should say what they do

Bad:

function addToDate(date, month) 
  // ...


const date = new Date();

// It's hard to tell from the function name what is added
addToDate(date, 1);

Good:

function addMonthToDate(month, date) 
  // ...


const date = new Date();
addMonthToDate(1, date);

Functions should only be one level of abstraction

When you have more than one level of abstraction your function is usually doing too much. Splitting up functions leads to reusability and easier testing.

Bad:

function parseBetterJSAlternative(code) 
  const REGEXES = [
    // ...
  ];

  const statements = code.split(" ");
  const tokens = [];
  REGEXES.forEach(REGEX => 
    statements.forEach(statement => 
      // ...
    );
  );

  const ast = [];
  tokens.forEach(token => 
    // lex...
  );

  ast.forEach(node => 
    // parse...
  );

Good:

function parseBetterJSAlternative(code) 
  const tokens = tokenize(code);
  const syntaxTree = parse(tokens);
  syntaxTree.forEach(node => 
    // parse...
  );


function tokenize(code) 
  const REGEXES = [
    // ...
  ];

  const statements = code.split(" ");
  const tokens = [];
  REGEXES.forEach(REGEX => 
    statements.forEach(statement => 
      tokens.push(/* ... */);
    );
  );

  return tokens;


function parse(tokens) 
  const syntaxTree = [];
  tokens.forEach(token => 
    syntaxTree.push(/* ... */);
  );

  return syntaxTree;

Remove duplicate code

Do your absolute best to avoid duplicate code. Duplicate code is bad because it means that there's more than one place to alter something if you need to change some logic.

Imagine if you run a restaurant and you keep track of your inventory: all your tomatoes, onions, garlic, spices, etc. If you have multiple lists that you keep this on, then all have to be updated when you serve a dish with tomatoes in them. If you only have one list, there's only one place to update!

Oftentimes you have duplicate code because you have two or more slightly different things, that share a lot in common, but their differences force you to have two or more separate functions that do much of the same things. Removing duplicate code means creating an abstraction that can handle this set of different things with just one function/module/class.

Getting the abstraction right is critical, that's why you should follow the SOLID principles laid out in the Classes section. Bad abstractions can be worse than duplicate code, so be careful! Having said this, if you can make a good abstraction, do it! Don't repeat yourself, otherwise you'll find yourself updating multiple places anytime you want to change one thing.

Bad:

function showDeveloperList(developers) 
  developers.forEach(developer => 
    const expectedSalary = developer.calculateExpectedSalary();
    const experience = developer.getExperience();
    const githubLink = developer.getGithubLink();
    const data = 
      expectedSalary,
      experience,
      githubLink
    ;

    render(data);
  );


function showManagerList(managers) 
  managers.forEach(manager => 
    const expectedSalary = manager.calculateExpectedSalary();
    const experience = manager.getExperience();
    const portfolio = manager.getMBAProjects();
    const data = 
      expectedSalary,
      experience,
      portfolio
    ;

    render(data);
  );

Good:

function showEmployeeList(employees) 
  employees.forEach(employee => 
    const expectedSalary = employee.calculateExpectedSalary();
    const experience = employee.getExperience();

    const data = 
      expectedSalary,
      experience
    ;

    switch (employee.type) 
      case "manager":
        data.portfolio = employee.getMBAProjects();
        break;
      case "developer":
        data.githubLink = employee.getGithubLink();
        break;
    

    render(data);
  );

Set default objects with Object.assign

Bad:

const menuConfig = 
  title: null,
  body: "Bar",
  buttonText: null,
  cancellable: true
;

function createMenu(config) 
  config.title = config.title || "Foo";
  config.body = config.body || "Bar";
  config.buttonText = config.buttonText || "Baz";
  config.cancellable =
    config.cancellable !== undefined ? config.cancellable : true;


createMenu(menuConfig);

Good:

const menuConfig = 
  title: "Order",
  // User did not include 'body' key
  buttonText: "Send",
  cancellable: true
;

function createMenu(config) 
  let finalConfig = Object.assign(
    
      title: "Foo",
      body: "Bar",
      buttonText: "Baz",
      cancellable: true
    ,
    config
  );
  return finalConfig
  // config now equals: title: "Order", body: "Bar", buttonText: "Send", cancellable: true
  // ...


createMenu(menuConfig);

Don't use flags as function parameters

Flags tell your user that this function does more than one thing. Functions should do one thing. Split out your functions if they are following different code paths based on a boolean.

Bad:

function createFile(name, temp) 
  if (temp) 
    fs.create(`./temp/$name`);
   else 
    fs.create(name);

Good:

function createFile(name) 
  fs.create(name);


function createTempFile(name) 
  createFile(`./temp/$name`);

Avoid Side Effects (part 1)

A function produces a side effect if it does anything other than take a value in and return another value or values. A side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a stranger.

Now, you do need to have side effects in a program on occasion. Like the previous example, you might need to write to a file. What you want to do is to centralize where you are doing this. Don't have several functions and classes that write to a particular file. Have one service that does it. One and only one.

The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, and not centralizing where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.

Bad:

// Global variable referenced by following function.
// If we had another function that used this name, now it'd be an array and it could break it.
let name = "Ryan McDermott";

function splitIntoFirstAndLastName() 
  name = name.split(" ");


splitIntoFirstAndLastName();

console.log(name); // ['Ryan', 'McDermott'];

Good:

function splitIntoFirstAndLastName(name) 
  return name.split(" ");


const name = "Ryan McDermott";
const newName = splitIntoFirstAndLastName(name);

console.log(name); // 'Ryan McDermott';
console.log(newName); // ['Ryan', 'McDermott'];

Avoid Side Effects (part 2)

In JavaScript, some values are unchangeable (immutable) and some are changeable (mutable). Objects and arrays are two kinds of mutable values so it's important to handle them carefully when they're passed as parameters to a function. A JavaScript function can change an object's properties or alter the contents of an array which could easily cause bugs elsewhere.

Suppose there's a function that accepts an array parameter representing a shopping cart. If the function makes a change in that shopping cart array - by adding an item to purchase, for example - then any other function that uses that same cart array will be affected by this addition. That may be great, however it could also be bad. Let's imagine a bad situation:

The user clicks the "Purchase" button which calls a purchase function that spawns a network request and sends the cart array to the server. Because of a bad network connection, the purchase function has to keep retrying the request. Now, what if in the meantime the user accidentally clicks an "Add to Cart" button on an item they don't actually want before the network request begins? If that happens and the network request begins, then that purchase function will send the accidentally added item because the cart array was modified.

A great solution would be for the addItemToCart function to always clone the cart, edit it, and return the clone. This would ensure that functions that are still using the old shopping cart wouldn't be affected by the changes.

Two caveats to mention to this approach:

  1. There might be cases where you actually want to modify the input object, but when you adopt this programming practice you will find that those cases are pretty rare. Most things can be refactored to have no side effects!

  2. Cloning big objects can be very expensive in terms of performance. Luckily, this isn't a big issue in practice because there are great libraries that allow this kind of programming approach to be fast and not as memory intensive as it would be for you to manually clone objects and arrays.

Bad:

const addItemToCart = (cart, item) => 
  cart.push( item, date: Date.now() );
;

Good:

const addItemToCart = (cart, item) => 
  return [...cart,  item, date: Date.now() ];
;

Don't write to global functions

Polluting globals is a bad practice in JavaScript because you could clash with another library and the user of your API would be none-the-wiser until they get an exception in production. Let's think about an example: what if you wanted to extend JavaScript's native Array method to have a diff method that could show the difference between two arrays? You could write your new function to the Array.prototype, but it could clash with another library that tried to do the same thing. What if that other library was just using diff to find the difference between the first and last elements of an array? This is why it would be much better to just use ES2015/ES6 classes and simply extend the Array global.

Bad:

Array.prototype.diff = function diff(comparisonArray) 
  const hash = new Set(comparisonArray);
  return this.filter(elem => !hash.has(elem));
;

Good:

class SuperArray extends Array 
  diff(comparisonArray) 
    const hash = new Set(comparisonArray);
    return this.filter(elem => !hash.has(elem));

Favor functional programming over imperative programming

JavaScript isn't a functional language in the way that Haskell is, but it has a functional flavor to it. Functional languages can be cleaner and easier to test. Favor this style of programming when you can.

Bad:

const programmerOutput = [
  
    name: "Uncle Bobby",
    linesOfCode: 500
  ,
  
    name: "Suzie Q",
    linesOfCode: 1500
  ,
  
    name: "Jimmy Gosling",
    linesOfCode: 150
  ,
  
    name: "Gracie Hopper",
    linesOfCode: 1000
  
];

let totalOutput = 0;

for (let i = 0; i < programmerOutput.length; i++) 
  totalOutput += programmerOutput[i].linesOfCode;

Good:

const programmerOutput = [
  
    name: "Uncle Bobby",
    linesOfCode: 500
  ,
  
    name: "Suzie Q",
    linesOfCode: 1500
  ,
  
    name: "Jimmy Gosling",
    linesOfCode: 150
  ,
  
    name: "Gracie Hopper",
    linesOfCode: 1000
  
];

const totalOutput = programmerOutput.reduce(
  (totalLines, output) => totalLines + output.linesOfCode,
  0
);

Encapsulate conditionals

Bad:

if (fsm.state === "fetching" && isEmpty(listNode)) 
  // ...

Good:

function shouldShowSpinner(fsm, listNode) 
  return fsm.state === "fetching" && isEmpty(listNode);


if (shouldShowSpinner(fsmInstance, listNodeInstance)) 
  // ...

Avoid negative conditionals

Bad:

function isDOMNodeNotPresent(node) 
  // ...


if (!isDOMNodeNotPresent(node)) 
  // ...

Good:

function isDOMNodePresent(node) 
  // ...


if (isDOMNodePresent(node)) 
  // ...

Avoid conditionals

This seems like an impossible task. Upon first hearing this, most people say, "how am I supposed to do anything without an if statement?" The answer is that you can use polymorphism to achieve the same task in many cases. The second question is usually, "well that's great but why would I want to do that?" The answer is a previous clean code concept we learned: a function should only do one thing. When you have classes and functions that have if statements, you are telling your user that your function does more than one thing. Remember, just do one thing.

Bad:

class Airplane 
  // ...
  getCruisingAltitude() 
    switch (this.type) 
      case "777":
        return this.getMaxAltitude() - this.getPassengerCount();
      case "Air Force One":
        return this.getMaxAltitude();
      case "Cessna":
        return this.getMaxAltitude() - this.getFuelExpenditure();

Good:

class Airplane 
  // ...


class Boeing777 extends Airplane 
  // ...
  getCruisingAltitude() 
    return this.getMaxAltitude() - this.getPassengerCount();
  


class AirForceOne extends Airplane 
  // ...
  getCruisingAltitude() 
    return this.getMaxAltitude();
  


class Cessna extends Airplane 
  // ...
  getCruisingAltitude() 
    return this.getMaxAltitude() - this.getFuelExpenditure();

Avoid type-checking (part 1)

JavaScript is untyped, which means your functions can take any type of argument. Sometimes you are bitten by this freedom and it becomes tempting to do type-checking in your functions. There are many ways to avoid having to do this. The first thing to consider is consistent APIs.

Bad:

function travelToTexas(vehicle) 
  if (vehicle instanceof Bicycle) 
    vehicle.pedal(this.currentLocation, new Location("texas"));
   else if (vehicle instanceof Car) 
    vehicle.drive(this.currentLocation, new Location("texas"));

Good:

function travelToTexas(vehicle) 
  vehicle.move(this.currentLocation, new Location("texas"));

Avoid type-checking (part 2)

If you are working with basic primitive values like strings and integers, and you can't use polymorphism but you still feel the need to type-check, you should consider using TypeScript. It is an excellent alternative to normal JavaScript, as it provides you with static typing on top of standard JavaScript syntax. The problem with manually type-checking normal JavaScript is that doing it well requires so much extra verbiage that the faux "type-safety" you get doesn't make up for the lost readability. Keep your JavaScript clean, write good tests, and have good code reviews. Otherwise, do all of that but with TypeScript (which, like I said, is a great alternative!).

Bad:

function combine(val1, val2) 
  if (
    (typeof val1 === "number" && typeof val2 === "number") ||
    (typeof val1 === "string" && typeof val2 === "string")
  ) 
    return val1 + val2;
  

  throw new Error("Must be of type String or Number");

Good:

function combine(val1, val2) 
  return val1 + val2;

Don't over-optimize

Modern browsers do a lot of optimization under-the-hood at runtime. A lot of times, if you are optimizing then you are just wasting your time. There are good resources for seeing where optimization is lacking. Target those in the meantime, until they are fixed if they can be.

Bad:

// On old browsers, each iteration with uncached `list.length` would be costly
// because of `list.length` recomputation. In modern browsers, this is optimized.
for (let i = 0, len = list.length; i < len; i++) 
  // ...

Good:

for (let i = 0; i < list.length; i++) 
  // ...

Remove dead code

Dead code is just as bad as duplicate code. There's no reason to keep it in your codebase. If it's not being called, get rid of it! It will still be safe in your version history if you still need it.

Bad:

function oldRequestModule(url) 
  // ...


function newRequestModule(url) 
  // ...


const req = newRequestModule;
inventoryTracker("apples", req, "www.inventory-awesome.io");

Good:

function newRequestModule(url) 
  // ...


const req = newRequestModule;
inventoryTracker("apples", req, "www.inventory-awesome.io");

Objects and Data Structures

Use getters and setters

Using getters and setters to access data on objects could be better than simply looking for a property on an object. "Why?" you might ask. Well, here's an unorganized list of reasons why:

  • When you want to do more beyond getting an object property, you don't have to look up and change every accessor in your codebase.

  • Makes adding validation simple when doing a set.

  • Encapsulates the internal representation.

  • Easy to add logging and error handling when getting and setting.

  • You can lazy load your object's properties, let's say getting it from a server.

Bad:

function makeBankAccount() 
  // ...

  return 
    balance: 0
    // ...
  ;


const account = makeBankAccount();
account.balance = 100;

Good:

function makeBankAccount() 
  // this one is private
  let balance = 0;

  // a "getter", made public via the returned object below
  function getBalance() 
    return balance;
  

  // a "setter", made public via the returned object below
  function setBalance(amount) 
    // ... validate before updating the balance
    balance = amount;
  

  return 
    // ...
    getBalance,
    setBalance
  ;


const account = makeBankAccount();
account.setBalance(100);

Make objects have private members

This can be accomplished through closures (for ES5 and below).

Bad:

const Employee = function(name) 
  this.name = name;
;

Employee.prototype.getName = function getName() 
  return this.name;
;

const employee = new Employee("John Doe");
console.log(`Employee name: $employee.getName()`); // Employee name: John Doe
delete employee.name;
console.log(`Employee name: $employee.getName()`); // Employee name: undefined

Good:

function makeEmployee(name) 
  return 
    getName() 
      return name;
    
  ;


const employee = makeEmployee("John Doe");
console.log(`Employee name: $employee.getName()`); // Employee name: John Doe
delete employee.name;
console.log(`Employee name: $employee.getName()`); // Employee name: John Doe

Classes

Prefer ES2015/ES6 classes over ES5 plain functions

It's very difficult to get readable class inheritance, construction, and method definitions for classical ES5 classes. If you need inheritance (and be aware that you might not), then prefer ES2015/ES6 classes. However, prefer small functions over classes until you find yourself needing larger and more complex objects.

Bad:

const Animal = function(age) 
  if (!(this instanceof Animal)) 
    throw new Error("Instantiate Animal with `new`");
  

  this.age = age;
;

Animal.prototype.move = function move() ;

const Mammal = function(age, furColor) 
  if (!(this instanceof Mammal)) 
    throw new Error("Instantiate Mammal with `new`");
  

  Animal.call(this, age);
  this.furColor = furColor;
;

Mammal.prototype = Object.create(Animal.prototype);
Mammal.prototype.constructor = Mammal;
Mammal.prototype.liveBirth = function liveBirth() ;

const Human = function(age, furColor, languageSpoken) 
  if (!(this instanceof Human)) 
    throw new Error("Instantiate Human with `new`");
  

  Mammal.call(this, age, furColor);
  this.languageSpoken = languageSpoken;
;

Human.prototype = Object.create(Mammal.prototype);
Human.prototype.constructor = Human;
Human.prototype.speak = function speak() ;

Good:

class Animal 
  constructor(age) 
    this.age = age;
  

  move() 
    /* ... */
  


class Mammal extends Animal 
  constructor(age, furColor) 
    super(age);
    this.furColor = furColor;
  

  liveBirth() 
    /* ... */
  


class Human extends Mammal 
  constructor(age, furColor, languageSpoken) 
    super(age, furColor);
    this.languageSpoken = languageSpoken;
  

  speak() 
    /* ... */

Use method chaining

This pattern is very useful in JavaScript and you see it in many libraries such as jQuery and Lodash. It allows your code to be expressive, and less verbose. For that reason, I say, use method chaining and take a look at how clean your code will be. In your class functions, simply return this at the end of every function, and you can chain further class methods onto it.

Bad:

class Car 
  constructor(make, model, color) 
    this.make = make;
    this.model = model;
    this.color = color;
  

  setMake(make) 
    this.make = make;
  

  setModel(model) 
    this.model = model;
  

  setColor(color) 
    this.color = color;
  

  save() 
    console.log(this.make, this.model, this.color);
  


const car = new Car("Ford", "F-150", "red");
car.setColor("pink");
car.save();

Good:

class Car 
  constructor(make, model, color) 
    this.make = make;
    this.model = model;
    this.color = color;
  

  setMake(make) 
    this.make = make;
    // NOTE: Returning this for chaining
    return this;
  

  setModel(model) 
    this.model = model;
    // NOTE: Returning this for chaining
    return this;
  

  setColor(color) 
    this.color = color;
    // NOTE: Returning this for chaining
    return this;
  

  save() 
    console.log(this.make, this.model, this.color);
    // NOTE: Returning this for chaining
    return this;
  


const car = new Car("Ford", "F-150", "red").setColor("pink").save();

Prefer composition over inheritance

As stated famously in Design Patterns by the Gang of Four, you should prefer composition over inheritance where you can. There are lots of good reasons to use inheritance and lots of good reasons to use composition. The main point for this maxim is that if your mind instinctively goes for inheritance, try to think if composition could model your problem better. In some cases it can.

You might be wondering then, "when should I use inheritance?" It depends on your problem at hand, but this is a decent list of when inheritance makes more sense than composition:

  1. Your inheritance represents an "is-a" relationship and not a "has-a" relationship (Human->Animal vs. User->UserDetails).

  2. You can reuse code from the base classes (Humans can move like all animals).

  3. You want to make global changes to derived classes by changing a base class. (Change the caloric expenditure of all animals when they move).

Bad:

class Employee 
  constructor(name, email) 
    this.name = name;
    this.email = email;
  

  // ...


// Bad because Employees "have" tax data. EmployeeTaxData is not a type of Employee
class EmployeeTaxData extends Employee 
  constructor(ssn, salary) 
    super();
    this.ssn = ssn;
    this.salary = salary;
  

  // ...

Good:
class EmployeeTaxData 
  constructor(ssn, salary) 
    this.ssn = ssn;
    this.salary = salary;
  

  // ...


class Employee 
  constructor(name, email) 
    this.name = name;
    this.email = email;
  

  setTaxData(ssn, salary) 
    this.taxData = new EmployeeTaxData(ssn, salary);
  
  // ...

SOLID

Single Responsibility Principle (SRP)

As stated in Clean Code, "There should never be more than one reason for a class to change". It's tempting to jam-pack a class with a lot of functionality, like when you can only take one suitcase on your flight. The issue with this is that your class won't be conceptually cohesive and it will give it many reasons to change. Minimizing the amount of times you need to change a class is important. It's important because if too much functionality is in one class and you modify a piece of it, it can be difficult to understand how that will affect other dependent modules in your codebase.

Bad:

class UserSettings 
  constructor(user) 
    this.user = user;
  

  changeSettings(settings) 
    if (this.verifyCredentials()) 
      // ...
    
  

  verifyCredentials() 
    // ...

Good:

class UserAuth 
  constructor(user) 
    this.user = user;
  

  verifyCredentials() 
    // ...
  


class UserSettings 
  constructor(user) 
    this.user = user;
    this.auth = new UserAuth(user);
  

  changeSettings(settings) 
    if (this.auth.verifyCredentials()) 
      // ...

Open/Closed Principle (OCP)

As stated by Bertrand Meyer, "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification." What does that mean though? This principle basically states that you should allow users to add new functionalities without changing existing code.

Bad:

class AjaxAdapter extends Adapter 
  constructor() 
    super();
    this.name = "ajaxAdapter";
  


class NodeAdapter extends Adapter 
  constructor() 
    super();
    this.name = "nodeAdapter";
  


class HttpRequester 
  constructor(adapter) 
    this.adapter = adapter;
  

  fetch(url) 
    if (this.adapter.name === "ajaxAdapter") 
      return makeAjaxCall(url).then(response => 
        // transform response and return
      );
     else if (this.adapter.name === "nodeAdapter") 
      return makeHttpCall(url).then(response => 
        // transform response and return
      );
    
  


function makeAjaxCall(url) 
  // request and return promise


function makeHttpCall(url) 
  // request and return promise

Good:

class AjaxAdapter extends Adapter 
  constructor() 
    super();
    this.name = "ajaxAdapter";
  

  request(url) 
    // request and return promise
  


class NodeAdapter extends Adapter 
  constructor() 
    super();
    this.name = "nodeAdapter";
  

  request(url) 
    // request and return promise
  


class HttpRequester 
  constructor(adapter) 
    this.adapter = adapter;
  

  fetch(url) 
    return this.adapter.request(url).then(response => 
      // transform response and return
    );

Liskov Substitution Principle (LSP)

This is a scary term for a very simple concept. It's formally defined as "If S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may substitute objects of type T) without altering any of the desirable properties of that program (correctness, task performed, etc.)." That's an even scarier definition.

The best explanation for this is if you have a parent class and a child class, then the base class and child class can be used interchangeably without getting incorrect results. This might still be confusing, so let's take a look at the classic Square-Rectangle example. Mathematically, a square is a rectangle, but if you model it using the "is-a" relationship via inheritance, you quickly get into trouble.

Bad:

class Rectangle 
  constructor() 
    this.width = 0;
    this.height = 0;
  

  setColor(color) 
    // ...
  

  render(area) 
    // ...
  

  setWidth(width) 
    this.width = width;
  

  setHeight(height) 
    this.height = height;
  

  getArea() 
    return this.width * this.height;
  


class Square extends Rectangle 
  setWidth(width) 
    this.width = width;
    this.height = width;
  

  setHeight(height) 
    this.width = height;
    this.height = height;
  


function renderLargeRectangles(rectangles) 
  rectangles.forEach(rectangle => 
    rectangle.setWidth(4);
    rectangle.setHeight(5);
    const area = rectangle.getArea(); // BAD: Returns 25 for Square. Should be 20.
    rectangle.render(area);
  );


const rectangles = [new Rectangle(), new Rectangle(), new Square()];
renderLargeRectangles(rectangles);

Good:

class Shape 
  setColor(color) 
    // ...
  

  render(area) 
    // ...
  


class Rectangle extends Shape 
  constructor(width, height) 
    super();
    this.width = width;
    this.height = height;
  

  getArea() 
    return this.width * this.height;
  


class Square extends Shape 
  constructor(length) 
    super();
    this.length = length;
  

  getArea() 
    return this.length * this.length;
  


function renderLargeShapes(shapes) 
  shapes.forEach(shape => 
    const area = shape.getArea();
    shape.render(area);
  );


const shapes = [new Rectangle(4, 5), new Rectangle(4, 5), new Square(5)];
renderLargeShapes(shapes);

Interface Segregation Principle (ISP)

JavaScript doesn't have interfaces so this principle doesn't apply as strictly as others. However, it's important and relevant even with JavaScript's lack of type system.

ISP states that "Clients should not be forced to depend upon interfaces that they do not use." Interfaces are implicit contracts in JavaScript because of duck typing.

A good example to look at that demonstrates this principle in JavaScript is for classes that require large settings objects. Not requiring clients to setup huge amounts of options is beneficial, because most of the time they won't need all of the settings. Making them optional helps prevent having a "fat interface".

Bad:

class DOMTraverser 
  constructor(settings) 
    this.settings = settings;
    this.setup();
  

  setup() 
    this.rootNode = this.settings.rootNode;
    this.settings.animationModule.setup();
  

  traverse() 
    // ...
  


const $ = new DOMTraverser(
  rootNode: document.getElementsByTagName("body"),
  animationModule()  // Most of the time, we won't need to animate when traversing.
  // ...
);

Good:

class DOMTraverser 
  constructor(settings) 
    this.settings = settings;
    this.options = settings.options;
    this.setup();
  

  setup() 
    this.rootNode = this.settings.rootNode;
    this.setupOptions();
  

  setupOptions() 
    if (this.options.animationModule) 
      // ...
    
  

  traverse() 
    // ...
  


const $ = new DOMTraverser(
  rootNode: document.getElementsByTagName("body"),
  options: 
    animationModule() 
  
);

Dependency Inversion Principle (DIP)

This principle states two essential things:

  1. High-level modules should not depend on low-level modules. Both should depend on abstractions.

  2. Abstractions should not depend upon details. Details should depend on abstractions.

This can be hard to understand at first, but if you've worked with AngularJS, you've seen an implementation of this principle in the form of Dependency Injection (DI). While they are not identical concepts, DIP keeps high-level modules from knowing the details of its low-level modules and setting them up. It can accomplish this through DI. A huge benefit of this is that it reduces the coupling between modules. Coupling is a very bad development pattern because it makes your code hard to refactor.

As stated previously, JavaScript doesn't have interfaces so the abstractions that are depended upon are implicit contracts. That is to say, the methods and properties that an object/class exposes to another object/class. In the example below, the implicit contract is that any Request module for an InventoryTracker will have a requestItems method.

Bad:

class InventoryRequester 
  constructor() 
    this.REQ_METHODS = ["HTTP"];
  

  requestItem(item) 
    // ...
  


class InventoryTracker 
  constructor(items) 
    this.items = items;

    // BAD: We have created a dependency on a specific request implementation.
    // We should just have requestItems depend on a request method: `request`
    this.requester = new InventoryRequester();
  

  requestItems() 
    this.items.forEach(item => 
      this.requester.requestItem(item);
    );
  


const inventoryTracker = new InventoryTracker(["apples", "bananas"]);
inventoryTracker.requestItems();

Good:

class InventoryTracker 
  constructor(items, requester) 
    this.items = items;
    this.requester = requester;
  

  requestItems() 
    this.items.forEach(item => 
      this.requester.requestItem(item);
    );
  


class InventoryRequesterV1 
  constructor() 
    this.REQ_METHODS = ["HTTP"];
  

  requestItem(item) 
    // ...
  


class InventoryRequesterV2 
  constructor() 
    this.REQ_METHODS = ["WS"];
  

  requestItem(item) 
    // ...
  


// By constructing our dependencies externally and injecting them, we can easily
// substitute our request module for a fancy new one that uses WebSockets.
const inventoryTracker = new InventoryTracker(
  ["apples", "bananas"],
  new InventoryRequesterV2()
);
inventoryTracker.requestItems();

Testing

Testing is more important than shipping. If you have no tests or an inadequate amount, then every time you ship code you won't be sure that you didn't break anything. Deciding on what constitutes an adequate amount is up to your team, but having 100% coverage (all statements and branches) is how you achieve very high confidence and developer peace of mind. This means that in addition to having a great testing framework, you also need to use a good coverage tool.

There's no excuse to not write tests. There are plenty of good JS test frameworks, so find one that your team prefers. When you find one that works for your team, then aim to always write tests for every new feature/module you introduce. If your preferred method is Test Driven Development (TDD), that is great, but the main point is to just make sure you are reaching your coverage goals before launching any feature, or refactoring an existing one.

Single concept per test

Bad:

import assert from "assert";

describe("MomentJS", () => 
  it("handles date boundaries", () => 
    let date;

    date = new MomentJS("1/1/2015");
    date.addDays(30);
    assert.equal("1/31/2015", date);

    date = new MomentJS("2/1/2016");
    date.addDays(28);
    assert.equal("02/29/2016", date);

    date = new MomentJS("2/1/2015");
    date.addDays(28);
    assert.equal("03/01/2015", date);
  );
);

Good:

import assert from "assert";

describe("MomentJS", () => 
  it("handles 30-day months", () => 
    const date = new MomentJS("1/1/2015");
    date.addDays(30);
    assert.equal("1/31/2015", date);
  );

  it("handles leap year", () => 
    const date = new MomentJS("2/1/2016");
    date.addDays(28);
    assert.equal("02/29/2016", date);
  );

  it("handles non-leap year", () => 
    const date = new MomentJS("2/1/2015");
    date.addDays(28);
    assert.equal("03/01/2015", date);
  );
);

Concurrency

Use Promises, not callbacks

Callbacks aren't clean, and they cause excessive amounts of nesting. With ES2015/ES6, Promises are a built-in global type. Use them!

Bad:

import  get  from "request";
import  writeFile  from "fs";

get(
  "https://en.wikipedia.org/wiki/Robert_Cecil_Martin",
  (requestErr, response, body) => 
    if (requestErr) 
      console.error(requestErr);
     else 
      writeFile("article.html", body, writeErr => 
        if (writeErr) 
          console.error(writeErr);
         else 
          console.log("File written");
        
      );
    
  
);

Good:

import  get  from "request-promise";
import  writeFile  from "fs-extra";

get("https://en.wikipedia.org/wiki/Robert_Cecil_Martin")
  .then(body => 
    return writeFile("article.html", body);
  )
  .then(() => 
    console.log("File written");
  )
  .catch(err => 
    console.error(err);
  );

Async/Await are even cleaner than Promises

Promises are a very clean alternative to callbacks, but ES2017/ES8 brings async and await which offer an even cleaner solution. All you need is a function that is prefixed in an async keyword, and then you can write your logic imperatively without a then chain of functions. Use this if you can take advantage of ES2017/ES8 features today!

Bad:

import  get  from "request-promise";
import  writeFile  from "fs-extra";

get("https://en.wikipedia.org/wiki/Robert_Cecil_Martin")
  .then(body => 
    return writeFile("article.html", body);
  )
  .then(() => 
    console.log("File written");
  )
  .catch(err => 
    console.error(err);
  );

Good:

import  get  from "request-promise";
import  writeFile  from "fs-extra";

async function getCleanCodeArticle() 
  try 
    const body = await get(
      "https://en.wikipedia.org/wiki/Robert_Cecil_Martin"
    );
    await writeFile("article.html", body);
    console.log("File written");
   catch (err) 
    console.error(err);
  


getCleanCodeArticle()

Error Handling

Thrown errors are a good thing! They mean the runtime has successfully identified when something in your program has gone wrong and it's letting you know by stopping function execution on the current stack, killing the process (in Node), and notifying you in the console with a stack trace.

Don't ignore caught errors

Doing nothing with a caught error doesn't give you the ability to ever fix or react to said error. Logging the error to the console (console.log) isn't much better as often times it can get lost in a sea of things printed to the console. If you wrap any bit of code in a try/catch it means you think an error may occur there and therefore you should have a plan, or create a code path, for when it occurs.

Bad:

try 
  functionThatMightThrow();
 catch (error) 
  console.log(error);

Good:

try 
  functionThatMightThrow();
 catch (error) 
  // One option (more noisy than console.log):
  console.error(error);
  // Another option:
  notifyUserOfError(error);
  // Another option:
  reportErrorToService(error);
  // OR do all three!

Don't ignore rejected promises

For the same reason you shouldn't ignore caught errors from try/catch.

Bad:

getdata()
  .then(data => 
    functionThatMightThrow(data);
  )
  .catch(error => 
    console.log(error);
  );

Good:

getdata()
  .then(data => 
    functionThatMightThrow(data);
  )
  .catch(error => 
    // One option (more noisy than console.log):
    console.error(error);
    // Another option:
    notifyUserOfError(error);
    // Another option:
    reportErrorToService(error);
    // OR do all three!
  );

Formatting

Formatting is subjective. Like many rules herein, there is no hard and fast rule that you must follow. The main point is DO NOT ARGUE over formatting. There are tons of tools to automate this. Use one! It's a waste of time and money for engineers to argue over formatting.

For things that don't fall under the purview of automatic formatting (indentation, tabs vs. spaces, double vs. single quotes, etc.) look here for some guidance.

Use consistent capitalization

JavaScript is untyped, so capitalization tells you a lot about your variables, functions, etc. These rules are subjective, so your team can choose whatever they want. The point is, no matter what you all choose, just be consistent.

Bad:

const DAYS_IN_WEEK = 7;
const daysInMonth = 30;

const songs = ["Back In Black", "Stairway to Heaven", "Hey Jude"];
const Artists = ["ACDC", "Led Zeppelin", "The Beatles"];

function eraseDatabase() 
function restore_database() 

class animal 
class Alpaca

Good:

const DAYS_IN_WEEK = 7;
const DAYS_IN_MONTH = 30;

const SONGS = ["Back In Black", "Stairway to Heaven", "Hey Jude"];
const ARTISTS = ["ACDC", "Led Zeppelin", "The Beatles"];

function eraseDatabase() 
function restoreDatabase() 

class Animal 
class Alpaca

Function callers and callees should be close

If a function calls another, keep those functions vertically close in the source file. Ideally, keep the caller right above the callee. We tend to read code from top-to-bottom, like a newspaper. Because of this, make your code read that way.

Bad:

class PerformanceReview 
  constructor(employee) 
    this.employee = employee;
  

  lookupPeers(
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