eslint/docs/rules/no-promise-executor-return.md

   1 # Disallow returning values from Promise executor functions (no-promise-executor-return)
   2
   3 The `new Promise` constructor accepts a single argument, called an *executor*.
   4
   5 ```js
   6 const myPromise = new Promise(function executor(resolve, reject) {
   7     readFile('foo.txt', function(err, result) {
   8         if (err) {
   9             reject(err);
  10         } else {
  11             resolve(result);
  12         }
  13     });
  14 });
  15 ```
  16
  17 The executor function usually initiates some asynchronous operation. Once it is finished, the executor should call `resolve` with the result, or `reject` if an error occurred.
  18
  19 The return value of the executor is ignored. Returning a value from an executor function is a possible error because the returned value cannot be used and it doesn't affect the promise in any way.
  20
  21 ## Rule Details
  22
  23 This rule disallows returning values from Promise executor functions.
  24
  25 Only `return` without a value is allowed, as it's a control flow statement.
  26
  27 Examples of **incorrect** code for this rule:
  28
  29 ```js
  30 /*eslint no-promise-executor-return: "error"*/
  31
  32 new Promise((resolve, reject) => {
  33     if (someCondition) {
  34         return defaultResult;
  35     }
  36     getSomething((err, result) => {
  37         if (err) {
  38             reject(err);
  39         } else {
  40             resolve(result);
  41         }
  42     });
  43 });
  44
  45 new Promise((resolve, reject) => getSomething((err, data) => {
  46     if (err) {
  47         reject(err);
  48     } else {
  49         resolve(data);
  50     }
  51 }));
  52
  53 new Promise(() => {
  54     return 1;
  55 });
  56 ```
  57
  58 Examples of **correct** code for this rule:
  59
  60 ```js
  61 /*eslint no-promise-executor-return: "error"*/
  62
  63 new Promise((resolve, reject) => {
  64     if (someCondition) {
  65         resolve(defaultResult);
  66         return;
  67     }
  68     getSomething((err, result) => {
  69         if (err) {
  70             reject(err);
  71         } else {
  72             resolve(result);
  73         }
  74     });
  75 });
  76
  77 new Promise((resolve, reject) => {
  78     getSomething((err, data) => {
  79         if (err) {
  80             reject(err);
  81         } else {
  82             resolve(data);
  83         }
  84     });
  85 });
  86
  87 Promise.resolve(1);
  88 ```
  89
  90 ## Further Reading
  91
  92 * [MDN Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)
  93
  94 ## Related Rules
  95
  96 * [no-async-promise-executor](no-async-promise-executor.md)