eslint/docs/rules/no-unused-expressions.md

   1 # Disallow Unused Expressions (no-unused-expressions)
   2
   3 An unused expression which has no effect on the state of the program indicates a logic error.
   4
   5 For example, `n + 1;` is not a syntax error, but it might be a typing mistake where a programmer meant an assignment statement `n += 1;` instead. Sometimes, such unused expressions may be eliminated by some build tools in production environment, which possibly breaks application logic.
   6
   7 ## Rule Details
   8
   9 This rule aims to eliminate unused expressions which have no effect on the state of the program.
  10
  11 This rule does not apply to function calls or constructor calls with the `new` operator, because they could have *side effects* on the state of the program.
  12
  13 ```js
  14 var i = 0;
  15 function increment() { i += 1; }
  16 increment(); // return value is unused, but i changed as a side effect
  17
  18 var nThings = 0;
  19 function Thing() { nThings += 1; }
  20 new Thing(); // constructed object is unused, but nThings changed as a side effect
  21 ```
  22
  23 This rule does not apply to directives (which are in the form of literal string expressions such as `"use strict";` at the beginning of a script, module, or function).
  24
  25 Sequence expressions (those using a comma, such as `a = 1, b = 2`) are always considered unused unless their return value is assigned or used in a condition evaluation, or a function call is made with the sequence expression value.
  26
  27 ## Options
  28
  29 This rule, in its default state, does not require any arguments. If you would like to enable one or more of the following you may pass an object with the options set as follows:
  30
  31 * `allowShortCircuit` set to `true` will allow you to use short circuit evaluations in your expressions (Default: `false`).
  32 * `allowTernary` set to `true` will enable you to use ternary operators in your expressions similarly to short circuit evaluations (Default: `false`).
  33 * `allowTaggedTemplates` set to `true` will enable you to use tagged template literals in your expressions (Default: `false`).
  34 * `enforceForJSX` set to `true` will flag unused JSX element expressions (Default: `false`).
  35
  36 These options allow unused expressions *only if all* of the code paths either directly change the state (for example, assignment statement) or could have *side effects* (for example, function call).
  37
  38 Examples of **incorrect** code for the default `{ "allowShortCircuit": false, "allowTernary": false }` options:
  39
  40 ```js
  41 /*eslint no-unused-expressions: "error"*/
  42
  43 0
  44
  45 if(0) 0
  46
  47 {0}
  48
  49 f(0), {}
  50
  51 a && b()
  52
  53 a, b()
  54
  55 c = a, b;
  56
  57 a() && function namedFunctionInExpressionContext () {f();}
  58
  59 (function anIncompleteIIFE () {});
  60
  61 injectGlobal`body{ color: red; }`
  62
  63 ```
  64
  65 Note that one or more string expression statements (with or without semi-colons) will only be considered as unused if they are not in the beginning of a script, module, or function (alone and uninterrupted by other statements). Otherwise, they will be treated as part of a "directive prologue", a section potentially usable by JavaScript engines. This includes "strict mode" directives.
  66
  67 ```js
  68 "use strict";
  69 "use asm"
  70 "use stricter";
  71 "use babel"
  72 "any other strings like this in the prologue";
  73 ```
  74
  75 Examples of **correct** code for the default `{ "allowShortCircuit": false, "allowTernary": false }` options:
  76
  77 ```js
  78 /*eslint no-unused-expressions: "error"*/
  79
  80 {} // In this context, this is a block statement, not an object literal
  81
  82 {myLabel: someVar} // In this context, this is a block statement with a label and expression, not an object literal
  83
  84 function namedFunctionDeclaration () {}
  85
  86 (function aGenuineIIFE () {}());
  87
  88 f()
  89
  90 a = 0
  91
  92 new C
  93
  94 delete a.b
  95
  96 void a
  97 ```
  98
  99 ### allowShortCircuit
 100
 101 Examples of **incorrect** code for the `{ "allowShortCircuit": true }` option:
 102
 103 ```js
 104 /*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/
 105
 106 a || b
 107 ```
 108
 109 Examples of **correct** code for the `{ "allowShortCircuit": true }` option:
 110
 111 ```js
 112 /*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/
 113
 114 a && b()
 115 a() || (b = c)
 116 ```
 117
 118 ### allowTernary
 119
 120 Examples of **incorrect** code for the `{ "allowTernary": true }` option:
 121
 122 ```js
 123 /*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/
 124
 125 a ? b : 0
 126 a ? b : c()
 127 ```
 128
 129 Examples of **correct** code for the `{ "allowTernary": true }` option:
 130
 131 ```js
 132 /*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/
 133
 134 a ? b() : c()
 135 a ? (b = c) : d()
 136 ```
 137
 138 ### allowShortCircuit and allowTernary
 139
 140 Examples of **correct** code for the `{ "allowShortCircuit": true, "allowTernary": true }` options:
 141
 142 ```js
 143 /*eslint no-unused-expressions: ["error", { "allowShortCircuit": true, "allowTernary": true }]*/
 144
 145 a ? b() || (c = d) : e()
 146 ```
 147
 148 ### allowTaggedTemplates
 149
 150 Examples of **incorrect** code for the `{ "allowTaggedTemplates": true }` option:
 151
 152 ```js
 153 /*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/
 154
 155 `some untagged template string`;
 156 ```
 157
 158 Examples of **correct** code for the `{ "allowTaggedTemplates": true }` option:
 159
 160 ```js
 161 /*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/
 162
 163 tag`some tagged template string`;
 164 ```
 165
 166 ### enforceForJSX
 167
 168 JSX is most-commonly used in the React ecosystem, where it is compiled to `React.createElement` expressions. Though free from side-effects, these calls are not automatically flagged by the `no-unused-expression` rule. If you're using React, or any other side-effect-free JSX pragma, this option can be enabled to flag these expressions.
 169
 170 Examples of **incorrect** code for the `{ "enforceForJSX": true }` option:
 171
 172 ```jsx
 173 /*eslint no-unused-expressions: ["error", { "enforceForJSX": true }]*/
 174
 175 <MyComponent />;
 176
 177 <></>;
 178 ```
 179
 180 Examples of **correct** code for the `{ "enforceForJSX": true }` option:
 181
 182 ```jsx
 183 /*eslint no-unused-expressions: ["error", { "enforceForJSX": true }]*/
 184
 185 var myComponentPartial = <MyComponent />;
 186
 187 var myFragment = <></>;
 188 ```