eslint/docs/rules/require-atomic-updates.md

   1 # Disallow assignments that can lead to race conditions due to usage of `await` or `yield` (require-atomic-updates)
   2
   3 When writing asynchronous code, it is possible to create subtle race condition bugs. Consider the following example:
   4
   5 ```js
   6 let totalLength = 0;
   7
   8 async function addLengthOfSinglePage(pageNum) {
   9   totalLength += await getPageLength(pageNum);
  10 }
  11
  12 Promise.all([addLengthOfSinglePage(1), addLengthOfSinglePage(2)]).then(() => {
  13   console.log('The combined length of both pages is', totalLength);
  14 });
  15 ```
  16
  17 This code looks like it will sum the results of calling `getPageLength(1)` and `getPageLength(2)`, but in reality the final value of `totalLength` will only be the length of one of the two pages. The bug is in the statement `totalLength += await getPageLength(pageNum);`. This statement first reads an initial value of `totalLength`, then calls `getPageLength(pageNum)` and waits for that Promise to fulfill. Finally, it sets the value of `totalLength` to the sum of `await getPageLength(pageNum)` and the *initial* value of `totalLength`. If the `totalLength` variable is updated in a separate function call during the time that the `getPageLength(pageNum)` Promise is pending, that update will be lost because the new value is overwritten without being read.
  18
  19 One way to fix this issue would be to ensure that `totalLength` is read at the same time as it's updated, like this:
  20
  21 ```js
  22 async function addLengthOfSinglePage(pageNum) {
  23   const lengthOfThisPage = await getPageLength(pageNum);
  24
  25   totalLength += lengthOfThisPage;
  26 }
  27 ```
  28
  29 Another solution would be to avoid using a mutable variable reference at all:
  30
  31 ```js
  32 Promise.all([getPageLength(1), getPageLength(2)]).then(pageLengths => {
  33   const totalLength = pageLengths.reduce((accumulator, length) => accumulator + length, 0);
  34
  35   console.log('The combined length of both pages is', totalLength);
  36 });
  37 ```
  38
  39 ## Rule Details
  40
  41 This rule aims to report assignments to variables or properties in cases where the assignments may be based on outdated values.
  42
  43 ### Variables
  44
  45 This rule reports an assignment to a variable when it detects the following execution flow in a generator or async function:
  46
  47 1. The variable is read.
  48 2. A `yield` or `await` pauses the function.
  49 3. After the function is resumed, a value is assigned to the variable from step 1.
  50
  51 The assignment in step 3 is reported because it may be incorrectly resolved because the value of the variable from step 1 may have changed between steps 2 and 3. In particular, if the variable can be accessed from other execution contexts (for example, if it is not a local variable and therefore other functions can change it), the value of the variable may have changed elsewhere while the function was paused in step 2.
  52
  53 Note that the rule does not report the assignment in step 3 in any of the following cases:
  54
  55 * If the variable is read again between steps 2 and 3.
  56 * If the variable cannot be accessed while the function is paused (for example, if it's a local variable).
  57
  58 Examples of **incorrect** code for this rule:
  59
  60 ```js
  61 /* eslint require-atomic-updates: error */
  62
  63 let result;
  64
  65 async function foo() {
  66     result += await something;
  67 }
  68
  69 async function bar() {
  70     result = result + await something;
  71 }
  72
  73 async function baz() {
  74     result = result + doSomething(await somethingElse);
  75 }
  76
  77 async function qux() {
  78     if (!result) {
  79         result = await initialize();
  80     }
  81 }
  82
  83 function* generator() {
  84     result += yield;
  85 }
  86 ```
  87
  88 Examples of **correct** code for this rule:
  89
  90 ```js
  91 /* eslint require-atomic-updates: error */
  92
  93 let result;
  94
  95 async function foobar() {
  96     result = await something + result;
  97 }
  98
  99 async function baz() {
 100     const tmp = doSomething(await somethingElse);
 101     result += tmp;
 102 }
 103
 104 async function qux() {
 105     if (!result) {
 106         const tmp = await initialize();
 107         if (!result) {
 108             result = tmp;
 109         }
 110     }
 111 }
 112
 113 async function quux() {
 114     let localVariable = 0;
 115     localVariable += await something;
 116 }
 117
 118 function* generator() {
 119     result = (yield) + result;
 120 }
 121 ```
 122
 123 ### Properties
 124
 125 This rule reports an assignment to a property through a variable when it detects the following execution flow in a generator or async function:
 126
 127 1. The variable or object property is read.
 128 2. A `yield` or `await` pauses the function.
 129 3. After the function is resumed, a value is assigned to a property.
 130
 131 This logic is similar to the logic for variables, but stricter because the property in step 3 doesn't have to be the same as the property in step 1. It is assumed that the flow depends on the state of the object as a whole.
 132
 133 Example of **incorrect** code for this rule:
 134
 135 ```js
 136 /* eslint require-atomic-updates: error */
 137
 138 async function foo(obj) {
 139     if (!obj.done) {
 140         obj.something = await getSomething();
 141     }
 142 }
 143 ```
 144
 145 Example of **correct** code for this rule:
 146
 147 ```js
 148 /* eslint require-atomic-updates: error */
 149
 150 async function foo(obj) {
 151     if (!obj.done) {
 152         const tmp = await getSomething();
 153         if (!obj.done) {
 154             obj.something = tmp;
 155         }
 156     }
 157 }
 158 ```
 159
 160 ## Options
 161
 162 This rule has an object option:
 163
 164 * `"allowProperties"`: When set to `true`, the rule does not report assignments to properties. Default is `false`.
 165
 166 ### allowProperties
 167
 168 Example of **correct** code for this rule with the `{ "allowProperties": true }` option:
 169
 170 ```js
 171 /* eslint require-atomic-updates: ["error", { "allowProperties": true }] */
 172
 173 async function foo(obj) {
 174     if (!obj.done) {
 175         obj.something = await getSomething();
 176     }
 177 }
 178 ```
 179
 180 ## When Not To Use It
 181
 182 If you don't use async or generator functions, you don't need to enable this rule.