javascript Javascript - cheatsheet

Posted 2021-05-03

if (age >= 14 && age < 19) {
  // logical condition
  status = "Eligible."; // executed if condition is true
} else {
  // else block is optional
  status = "Not eligible."; // executed if condition is false
}

// Switch Statement
switch (
  new Date().getDay() // input is current day
) {
  case 6: // if (day == 6)
    text = "Saturday";
    break;
  case 0: // if (day == 0)
    text = "Sunday";
    break;
  default:
    // else...
    text = "Whatever";
}

var a; // variable
var b = "init"; // string
var c = "Hi" + " " + "Joe"; // = "Hi Joe"
var d = 1 + 2 + "3"; // = "33"
var e = [2, 3, 5, 8]; // array
var f = false; // boolean
var g = /()/; // RegEx
var h = function() {}; // function object
const PI = 3.14; // constant
var a = 1,
  b = 2,
  c = a + b; // one line
let z = "zzz"; // block scope local variable

// Strict mode
"use strict"; // Use strict mode to write secure code
x = 1; // Throws an error because variable is not declared

// Values
false, true; // boolean
18, 3.14, 0b10011, 0xf6, NaN; // number
"flower", "John"; // string
undefined, null, Infinity; // special

// Operators
a = b + c - d; // addition, substraction
a = b * (c / d); // multiplication, division
x = 100 % 48; // modulo. 100 / 48 remainder = 4
a++;
b--; // postfix increment and decrement

// Bitwise operators
&	AND 	 5 & 1 (0101 & 0001)	1 (1)
|	OR 	 5 | 1 (0101 | 0001)	5 (101)
~	NOT 	 ~ 5 (~0101)	10 (1010)
^	XOR 	 5 ^ 1 (0101 ^ 0001)	4 (100)
<<	left shift 	 5 << 1 (0101 << 1)	10 (1010)
>>	right shift 	 5 >> 1 (0101 >> 1)	2 (10)
>>>	zero fill right shift 	 5 >>> 1 (0101 >>> 1)	2 (10)

// Arithmetic
a * (b + c)         // grouping
person.age          // member
person[age]         // member
!(a == b)           // logical not
a != b              // not equal
typeof a            // type (number, object, function...)
x << 2  x >> 3      // minary shifting
a = b               // assignment
a == b              // equals
a != b              // unequal
a === b             // strict equal
a !== b             // strict unequal
a < b   a > b       // less and greater than
a <= b  a >= b      // less or equal, greater or eq
a += b              // a = a + b (works with - * %...)
a && b              // logical and
a || b              // logical or

var age = 18; // number
var name = "Jane"; // string
var name = { first: "Jane", last: "Doe" }; // object
var truth = false; // boolean
var sheets = ["HTML", "CSS", "JS"]; // array
var a;
typeof a; // undefined
var a = null; // value null

// Objects
var student = {
  // object name
  firstName: "Jane", // list of properties and values
  lastName: "Doe",
  age: 18,
  height: 170,
  fullName: function() {
    // object function
    return this.firstName + " " + this.lastName;
  }
};
student.age = 19; // setting value
student[age]++; // incrementing
name = student.fullName(); // call object function

eval(); // executes a string as if it was script code
String(23); // return string from number
(23).toString(); // return string from number
Number("23"); // return number from string
decodeURI(enc); // decode URI. Result: "my page.asp"
encodeURI(uri); // encode URI. Result: "my%page.asp"
decodeURIComponent(enc); // decode a URI component
encodeURIComponent(uri); // encode a URI component
isFinite(); // is variable a finite, legal number
isNaN(); // is variable an illegal number
parseFloat(); // returns floating point number of string
parseInt(); // parses a string and returns an integer

var pi = 3.141;
pi.toFixed(0);          // returns 3
pi.toFixed(2);          // returns 3.14 - for working with money
pi.toPrecision(2)       // returns 3.1
pi.valueOf();           // returns number
Number(true);           // converts to number
Number(new Date())      // number of milliseconds since 1970
parseInt("3 months");   // returns the first number: 3
parseFloat("3.5 days"); // returns 3.5
Number.MAX_VALUE        // largest possible JS number
Number.MIN_VALUE        // smallest possible JS number
Number.NEGATIVE_INFINITY// -Infinity
Number.POSITIVE_INFINITY// Infinity
Math.
var pi = Math.PI;       // 3.141592653589793
Math.round(4.4);        // = 4 - rounded
Math.round(4.5);        // = 5
Math.pow(2,8);          // = 256 - 2 to the power of 8
Math.sqrt(49);          // = 7 - square root
Math.abs(-3.14);        // = 3.14 - absolute, positive value
Math.ceil(3.14);        // = 4 - rounded up
Math.floor(3.99);       // = 3 - rounded down
Math.sin(0);            // = 0 - sine
Math.cos(Math.PI);      // OTHERS: tan,atan,asin,acos,
Math.min(0, 3, -2, 2);  // = -2 - the lowest value
Math.max(0, 3, -2, 2);  // = 3 - the highest value
Math.log(1);            // = 0 natural logarithm
Math.exp(1);            // = 2.7182pow(E,x)
Math.random();          // random number between 0 and 1
Math.floor(Math.random() * 5) + 1;  // random integer, from 1 to 5
Constants like Math.PI:
E, PI, SQRT2, SQRT1_2, LN2, LN10, LOG2E, Log10E

var dogs = ["Bulldog", "Beagle", "Labrador"];
var dogs = new Array("Bulldog", "Beagle", "Labrador"); // declaration

alert(dogs[1]); // access value at index, first item being [0]
dogs[0] = "Bull Terier"; // change the first item

for (var i = 0; i < dogs.length; i++) {
  // parsing with array.length
  console.log(dogs[i]);
}
// Methods
dogs.toString(); // convert to string: results "Bulldog,Beagle,Labrador"
dogs.join(" * "); // join: "Bulldog * Beagle * Labrador"
dogs.pop(); // remove last element
dogs.push("Chihuahua"); // add new element to the end
dogs[dogs.length] = "Chihuahua"; // the same as push
dogs.shift(); // remove first element
dogs.unshift("Chihuahua"); // add new element to the beginning
delete dogs[0]; // change element to undefined (not recommended)
dogs.splice(2, 0, "Pug", "Boxer"); // add elements (where, how many to remove, element list)
var animals = dogs.concat(cats, birds); // join two arrays (dogs followed by cats and birds)
dogs.slice(1, 4); // elements from [1] to [4-1]
dogs.sort(); // sort string alphabetically
dogs.reverse(); // sort string in descending order
x.sort(function(a, b) {
  return a - b;
}); // numeric sort
x.sort(function(a, b) {
  return b - a;
}); // numeric descending sort
highest = x[0]; // first item in sorted array is the lowest (or highest) value
x.sort(function(a, b) {
  return 0.5 - Math.random();
}); // random order sort
// concat, copyWithin, every, fill, filter, find, findIndex, forEach, indexOf, isArray, join, lastIndexOf, map, pop, push, reduce, reduceRight, reverse, shift, slice, some, sort, splice, toString, unshift, valueOf