eslint/docs/rules/no-param-reassign.md

   1 # Disallow Reassignment of Function Parameters (no-param-reassign)
   2
   3 Assignment to variables declared as function parameters can be misleading and lead to confusing behavior, as modifying function parameters will also mutate the `arguments` object. Often, assignment to function parameters is unintended and indicative of a mistake or programmer error.
   4
   5 This rule can be also configured to fail when function parameters are modified. Side effects on parameters can cause counter-intuitive execution flow and make errors difficult to track down.
   6
   7 ## Rule Details
   8
   9 This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.
  10
  11 Examples of **incorrect** code for this rule:
  12
  13 ```js
  14 /*eslint no-param-reassign: "error"*/
  15
  16 function foo(bar) {
  17     bar = 13;
  18 }
  19
  20 function foo(bar) {
  21     bar++;
  22 }
  23
  24 function foo(bar) {
  25     for (bar in baz) {}
  26 }
  27
  28 function foo(bar) {
  29     for (bar of baz) {}
  30 }
  31 ```
  32
  33 Examples of **correct** code for this rule:
  34
  35 ```js
  36 /*eslint no-param-reassign: "error"*/
  37
  38 function foo(bar) {
  39     var baz = bar;
  40 }
  41 ```
  42
  43 ## Options
  44
  45 This rule takes one option, an object, with a boolean property `"props"`, and  arrays `"ignorePropertyModificationsFor"` and `"ignorePropertyModificationsForRegex"`. `"props"` is `false` by default. If `"props"` is set to `true`, this rule warns against the modification of parameter properties unless they're included in `"ignorePropertyModificationsFor"` or `"ignorePropertyModificationsForRegex"`, which is an empty array by default.
  46
  47 ### props
  48
  49 Examples of **correct** code for the default `{ "props": false }` option:
  50
  51 ```js
  52 /*eslint no-param-reassign: ["error", { "props": false }]*/
  53
  54 function foo(bar) {
  55     bar.prop = "value";
  56 }
  57
  58 function foo(bar) {
  59     delete bar.aaa;
  60 }
  61
  62 function foo(bar) {
  63     bar.aaa++;
  64 }
  65
  66 function foo(bar) {
  67     for (bar.aaa in baz) {}
  68 }
  69
  70 function foo(bar) {
  71     for (bar.aaa of baz) {}
  72 }
  73 ```
  74
  75 Examples of **incorrect** code for the `{ "props": true }` option:
  76
  77 ```js
  78 /*eslint no-param-reassign: ["error", { "props": true }]*/
  79
  80 function foo(bar) {
  81     bar.prop = "value";
  82 }
  83
  84 function foo(bar) {
  85     delete bar.aaa;
  86 }
  87
  88 function foo(bar) {
  89     bar.aaa++;
  90 }
  91
  92 function foo(bar) {
  93     for (bar.aaa in baz) {}
  94 }
  95
  96 function foo(bar) {
  97     for (bar.aaa of baz) {}
  98 }
  99 ```
 100
 101 Examples of **correct** code for the `{ "props": true }` option with `"ignorePropertyModificationsFor"` set:
 102
 103 ```js
 104 /*eslint no-param-reassign: ["error", { "props": true, "ignorePropertyModificationsFor": ["bar"] }]*/
 105
 106 function foo(bar) {
 107     bar.prop = "value";
 108 }
 109
 110 function foo(bar) {
 111     delete bar.aaa;
 112 }
 113
 114 function foo(bar) {
 115     bar.aaa++;
 116 }
 117
 118 function foo(bar) {
 119     for (bar.aaa in baz) {}
 120 }
 121
 122 function foo(bar) {
 123     for (bar.aaa of baz) {}
 124 }
 125 ```
 126
 127 Examples of **correct** code for the `{ "props": true }` option with `"ignorePropertyModificationsForRegex"` set:
 128
 129 ```js
 130 /*eslint no-param-reassign: ["error", { "props": true, "ignorePropertyModificationsForRegex": ["^bar"] }]*/
 131
 132 function foo(barVar) {
 133     barVar.prop = "value";
 134 }
 135
 136 function foo(barrito) {
 137     delete barrito.aaa;
 138 }
 139
 140 function foo(bar_) {
 141     bar_.aaa++;
 142 }
 143
 144 function foo(barBaz) {
 145     for (barBaz.aaa in baz) {}
 146 }
 147
 148 function foo(barBaz) {
 149     for (barBaz.aaa of baz) {}
 150 }
 151 ```
 152
 153
 154 ## When Not To Use It
 155
 156 If you want to allow assignment to function parameters, then you can safely disable this rule.
 157
 158 ## Further Reading
 159
 160 * [JavaScript: Don’t Reassign Your Function Arguments](https://spin.atomicobject.com/2011/04/10/javascript-don-t-reassign-your-function-arguments/)