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

   1 ---
   2 title: no-promise-executor-return
   3 layout: doc
   4 rule_type: problem
   5 related_rules:
   6 - no-async-promise-executor
   7 further_reading:
   8 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
   9 ---
  10
  11
  12 The `new Promise` constructor accepts a single argument, called an *executor*.
  13
  14 ```js
  15 const myPromise = new Promise(function executor(resolve, reject) {
  16     readFile('foo.txt', function(err, result) {
  17         if (err) {
  18             reject(err);
  19         } else {
  20             resolve(result);
  21         }
  22     });
  23 });
  24 ```
  25
  26 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.
  27
  28 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.
  29
  30 ## Rule Details
  31
  32 This rule disallows returning values from Promise executor functions.
  33
  34 Only `return` without a value is allowed, as it's a control flow statement.
  35
  36 Examples of **incorrect** code for this rule:
  37
  38 ::: incorrect
  39
  40 ```js
  41 /*eslint no-promise-executor-return: "error"*/
  42
  43 new Promise((resolve, reject) => {
  44     if (someCondition) {
  45         return defaultResult;
  46     }
  47     getSomething((err, result) => {
  48         if (err) {
  49             reject(err);
  50         } else {
  51             resolve(result);
  52         }
  53     });
  54 });
  55
  56 new Promise((resolve, reject) => getSomething((err, data) => {
  57     if (err) {
  58         reject(err);
  59     } else {
  60         resolve(data);
  61     }
  62 }));
  63
  64 new Promise(() => {
  65     return 1;
  66 });
  67 ```
  68
  69 :::
  70
  71 Examples of **correct** code for this rule:
  72
  73 ::: correct
  74
  75 ```js
  76 /*eslint no-promise-executor-return: "error"*/
  77
  78 new Promise((resolve, reject) => {
  79     if (someCondition) {
  80         resolve(defaultResult);
  81         return;
  82     }
  83     getSomething((err, result) => {
  84         if (err) {
  85             reject(err);
  86         } else {
  87             resolve(result);
  88         }
  89     });
  90 });
  91
  92 new Promise((resolve, reject) => {
  93     getSomething((err, data) => {
  94         if (err) {
  95             reject(err);
  96         } else {
  97             resolve(data);
  98         }
  99     });
 100 });
 101
 102 Promise.resolve(1);
 103 ```
 104
 105 :::