eslint/docs/rules/new-cap.md

   1 # require constructor names to begin with a capital letter (new-cap)
   2
   3 The `new` operator in JavaScript creates a new instance of a particular type of object. That type of object is represented by a constructor function. Since constructor functions are just regular functions, the only defining characteristic is that `new` is being used as part of the call. Native JavaScript functions begin with an uppercase letter to distinguish those functions that are to be used as constructors from functions that are not. Many style guides recommend following this pattern to more easily determine which functions are to be used as constructors.
   4
   5 ```js
   6 var friend = new Person();
   7 ```
   8
   9 ## Rule Details
  10
  11 This rule requires constructor names to begin with a capital letter. Certain built-in identifiers are exempt from this rule. These identifiers are:
  12
  13 * `Array`
  14 * `Boolean`
  15 * `Date`
  16 * `Error`
  17 * `Function`
  18 * `Number`
  19 * `Object`
  20 * `RegExp`
  21 * `String`
  22 * `Symbol`
  23
  24 Examples of **correct** code for this rule:
  25
  26 ```js
  27 /*eslint new-cap: "error"*/
  28
  29 function foo(arg) {
  30     return Boolean(arg);
  31 }
  32 ```
  33
  34 ## Options
  35
  36 This rule has an object option:
  37
  38 * `"newIsCap": true` (default) requires all `new` operators to be called with uppercase-started functions.
  39 * `"newIsCap": false` allows `new` operators to be called with lowercase-started or uppercase-started functions.
  40 * `"capIsNew": true` (default) requires all uppercase-started functions to be called with `new` operators.
  41 * `"capIsNew": false` allows uppercase-started functions to be called without `new` operators.
  42 * `"newIsCapExceptions"` allows specified lowercase-started function names to be called with the `new` operator.
  43 * `"newIsCapExceptionPattern"` allows any lowercase-started function names that match the specified regex pattern to be called with the `new` operator.
  44 * `"capIsNewExceptions"` allows specified uppercase-started function names to be called without the `new` operator.
  45 * `"capIsNewExceptionPattern"` allows any uppercase-started function names that match the specified regex pattern to be called without the `new` operator.
  46 * `"properties": true` (default) enables checks on object properties
  47 * `"properties": false` disables checks on object properties
  48
  49 ### newIsCap
  50
  51 Examples of **incorrect** code for this rule with the default `{ "newIsCap": true }` option:
  52
  53 ```js
  54 /*eslint new-cap: ["error", { "newIsCap": true }]*/
  55
  56 var friend = new person();
  57 ```
  58
  59 Examples of **correct** code for this rule with the default `{ "newIsCap": true }` option:
  60
  61 ```js
  62 /*eslint new-cap: ["error", { "newIsCap": true }]*/
  63
  64 var friend = new Person();
  65 ```
  66
  67 Examples of **correct** code for this rule with the `{ "newIsCap": false }` option:
  68
  69 ```js
  70 /*eslint new-cap: ["error", { "newIsCap": false }]*/
  71
  72 var friend = new person();
  73 ```
  74
  75 ### capIsNew
  76
  77 Examples of **incorrect** code for this rule with the default `{ "capIsNew": true }` option:
  78
  79 ```js
  80 /*eslint new-cap: ["error", { "capIsNew": true }]*/
  81
  82 var colleague = Person();
  83 ```
  84
  85 Examples of **correct** code for this rule with the default `{ "capIsNew": true }` option:
  86
  87 ```js
  88 /*eslint new-cap: ["error", { "capIsNew": true }]*/
  89
  90 var colleague = new Person();
  91 ```
  92
  93 Examples of **correct** code for this rule with the `{ "capIsNew": false }` option:
  94
  95 ```js
  96 /*eslint new-cap: ["error", { "capIsNew": false }]*/
  97
  98 var colleague = Person();
  99 ```
 100
 101 ### newIsCapExceptions
 102
 103 Examples of additional **correct** code for this rule with the `{ "newIsCapExceptions": ["events"] }` option:
 104
 105 ```js
 106 /*eslint new-cap: ["error", { "newIsCapExceptions": ["events"] }]*/
 107
 108 var events = require('events');
 109
 110 var emitter = new events();
 111 ```
 112
 113 ### newIsCapExceptionPattern
 114
 115 Examples of additional **correct** code for this rule with the `{ "newIsCapExceptionPattern": "^person\.." }` option:
 116
 117 ```js
 118 /*eslint new-cap: ["error", { "newIsCapExceptionPattern": "^person\.." }]*/
 119
 120 var friend = new person.acquaintance();
 121 var bestFriend = new person.friend();
 122 ```
 123
 124 ### capIsNewExceptions
 125
 126 Examples of additional **correct** code for this rule with the `{ "capIsNewExceptions": ["Person"] }` option:
 127
 128 ```js
 129 /*eslint new-cap: ["error", { "capIsNewExceptions": ["Person"] }]*/
 130
 131 function foo(arg) {
 132     return Person(arg);
 133 }
 134 ```
 135
 136 ### capIsNewExceptionPattern
 137
 138 Examples of additional **correct** code for this rule with the `{ "capIsNewExceptionPattern": "^Person\.." }` option:
 139
 140 ```js
 141 /*eslint new-cap: ["error", { "capIsNewExceptionPattern": "^Person\.." }]*/
 142
 143 var friend = person.Acquaintance();
 144 var bestFriend = person.Friend();
 145 ```
 146
 147 ### properties
 148
 149 Examples of **incorrect** code for this rule with the default `{ "properties": true }` option:
 150
 151 ```js
 152 /*eslint new-cap: ["error", { "properties": true }]*/
 153
 154 var friend = new person.acquaintance();
 155 ```
 156
 157 Examples of **correct** code for this rule with the default `{ "properties": true }` option:
 158
 159 ```js
 160 /*eslint new-cap: ["error", { "properties": true }]*/
 161
 162 var friend = new person.Acquaintance();
 163 ```
 164
 165 Examples of **correct** code for this rule with the `{ "properties": false }` option:
 166
 167 ```js
 168 /*eslint new-cap: ["error", { "properties": false }]*/
 169
 170 var friend = new person.acquaintance();
 171 ```
 172
 173 ## When Not To Use It
 174
 175 If you have conventions that don't require an uppercase letter for constructors, or don't require capitalized functions be only used as constructors, turn this rule off.