JS中的promise返回的resolve()和reject()的理解附代码展示

Posted 2023-04-27 bigShui

在JavaScript中，Promise是一种处理异步操作的方式。Promise对象有两个重要的方法：resolve()和reject()。

resolve()方法用于将Promise对象的状态从“未完成”变为“成功”，并将成功的结果作为参数传递给then()方法。例如：

let promise = new Promise(function(resolve, reject)
setTimeout(function()
resolve(\'Success!\');
, 1000);
);

promise.then(function(result)
console.log(result); // Success!
);

在上面的例子中，Promise对象在1秒后成功，并将字符串“Success!”作为参数传递给then()方法。

reject()方法用于将Promise对象的状态从“未完成”变为“失败”，并将失败的原因作为参数传递给catch()方法。例如：

let promise = new Promise(function(resolve, reject)
setTimeout(function()
reject(new Error(\'Something went wrong!\'));
, 1000);
);

promise.catch(function(error)
console.log(error); // Error: Something went wrong!
);

在上面的例子中，Promise对象在1秒后失败，并将一个Error对象作为参数传递给catch()方法。

总之，Promise对象的resolve()和reject()方法可以用于控制Promise对象的状态，并将成功或失败的结果传递给then()或catch()方法。

Promise 的 resolve 和 reject 函数的返回类型是啥？

【中文标题】Promise 的 resolve 和 reject 函数的返回类型是啥？

【英文标题】：What is the return type of resolve and reject function of a Promise?Promise 的 resolve 和 reject 函数的返回类型是什么？

【发布时间】：2017-11-09 01:39:01

【问题描述】：

我正在尝试查找我使用 typescript 创建的 Promise 的 resolve 和 reject 函数的返回类型，以便 tslint 不会根据 typedef 规则抱怨。

代码如下：

promises.ts

const promise: Promise<number> = new Promise((resolve, reject): void => 
    //the resolve / reject functions control the fate of the promise
    resolve(123);
);

//The promise fate can be subscribed to using .then (if resolved) or .catch (if rejected)
promise.then((res) => 
    console.log('I get called:', res === 123);      //I get called: true
);

---

tslint.json

    "defaultSeverity": "error",
    "extends": [
        "tslint:recommended"
    ],
    "jsRules": ,
    "rules": 
        "eofline": false,
        "no-console": false,
        "no-consecutive-blank-lines": false,
        "comment-format": false,
        "no-trailing-whitespace": false,
        "quotemark": false,
        "typedef": [true, "call-signature", "arrow-call-signature", "parameter", "arrow-parameter", "property-declaration", "variable-declaration", "member-variable-declaration"],
        "typedef-whitespace": false,
        "whitespace": false
    ,
    "rulesDirectory": []

---

tsconfig.json

  "compilerOptions": 
    /* Basic Options */                       
    "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd' or 'es2015'. */
    // "lib": [],                             /* Specify library files to be included in the compilation:  */
    // "allowJs": true,                       /* Allow javascript files to be compiled. */
    // "checkJs": true,                       /* Report errors in .js files. */
    // "jsx": "preserve",                     /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */
    // "declaration": true,                   /* Generates corresponding '.d.ts' file. */
    "sourceMap": true,                     /* Generates corresponding '.map' file. */
    // "outFile": "./",                       /* Concatenate and emit output to single file. */
     "outDir": "./build/",                        /* Redirect output structure to the directory. */
    // "rootDir": "./",                       /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
    // "removeComments": true,                /* Do not emit comments to output. */
    // "noEmit": true,                        /* Do not emit outputs. */
    // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
    // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
    // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */

    /* Strict Type-Checking Options */        
    "strict": true,                            /* Enable all strict type-checking options. */
    "noImplicitAny": true,                      /* Raise error on expressions and declarations with an implied 'any' type. */
    "strictNullChecks": true,              /* Enable strict null checks. */
    // "noImplicitThis": true,                /* Raise error on 'this' expressions with an implied 'any' type. */
    // "alwaysStrict": true,                  /* Parse in strict mode and emit "use strict" for each source file. */

    /* Additional Checks */                   
    // "noUnusedLocals": true,                /* Report errors on unused locals. */
    // "noUnusedParameters": true,            /* Report errors on unused parameters. */
    // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
    // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */

    /* Module Resolution Options */           
    // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
    // "baseUrl": "./",                       /* Base directory to resolve non-absolute module names. */
    // "paths": ,                           /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
    // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
    // "typeRoots": [],                       /* List of folders to include type definitions from. */
    // "types": [],                           /* Type declaration files to be included in compilation. */
    // "allowSyntheticDefaultImports": true,  /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */

    /* Source Map Options */                  
    // "sourceRoot": "./",                    /* Specify the location where debugger should locate TypeScript files instead of source locations. */
    // "mapRoot": "./",                       /* Specify the location where debugger should locate map files instead of generated locations. */
    // "inlineSourceMap": true,               /* Emit a single file with source maps instead of having a separate file. */
    // "inlineSources": true,                 /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */

    /* Experimental Options */                
    // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
    // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
    "lib": [
      "dom",
      "es5",
      "es2015.promise"
    ]
  

---

package.json

  "name": "TypescriptProject5",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": 
    "start": "webpack-dev-server",
    "build": "webpack --watch",
    "test": "echo \"Error: no test specified\" && exit 1"
  ,
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": 
    "ts-loader": "^2.1.0",
    "tslint": "^5.4.2",
    "typescript": "^2.3.4",
    "webpack": "^2.6.1",
    "webpack-dev-server": "^2.4.5",
    "write-file-webpack-plugin": "^4.0.2"
  ,
  "dependencies": 
    "es6-promise": "^4.1.0"
  

现在 tslint 抱怨 resolve、reject 和 res（预期的箭头参数有 typedef）

更新#2：这段代码没有给我任何 tslint 或 TS 错误

const promise: Promise<number> = new Promise((resolve: (value?:  | PromiseLike<> | undefined) => void, reject: (reason?: any) => void): void => 
    //the resolve / reject functions control the fate of the promise
    resolve(123);
);

//The promise fate can be subscribed to using .then (if resolved) or .catch (if rejected)
promise.then((res: number) => 
    console.log('I get called:', res === 123);      //I get called: true
);

【问题讨论】：

这是 tslint 的另一个禁止操作，但我可以在 tslint.json 中禁用此规则

我尝试使用函数声明，但它不适用于 res（代码中的第二条语句）

检查更新#2。当然，我总是可以按照您的建议将它们声明为 Function。您认为哪一个是最佳编码实践？

TypeScript编译器自动推断类型时，我认为描述参数的类型没有意义。

第二次更新到底有什么问题？

【参考方案1】：

这很容易测试。

new Promise(function (res, rej) 
    console.log(res());
);

new Promise(function (res, rej) 
    console.log(rej());
);

他们都返回undefined。

【讨论】：

如果我将解析参数定义为字符串，我会收到一条错误消息，指出此变量应该是值？： | PromiseLike |不明确的。我不知道这是否有帮助。

【参考方案2】：

您可以随时查看lib.d.ts file 中的定义，在这种情况下，PromiseConstructorLike 的定义是：

declare type PromiseConstructorLike = new <T>(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void) => PromiseLike<T>;

这意味着resolve的类型是：

(value?: T | PromiseLike<T>) => void

对于拒绝：

(reason?: any) => void

所以两者都返回void

【讨论】：

非常感谢您的快速回答，但 resolve 和 res 的类型定义不起作用。我遇到了几个错误。

换句话说使用 "noImplicitAny": false,