eslint/new-cap Style
What it does
This rule requires constructor names to begin with a capital letter.
Why is this bad?
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.
Warning: The option newIsCapExceptionPattern and capIsNewExceptionPattern are implemented with the rust regex syntax. Many JavaScript features are not supported (Lookahead, Lookbehinds, ...).
Examples
Examples of incorrect code for this rule:
js
function foo(arg) {
return Boolean(arg);
}Examples of incorrect code for this rule with the default { "newIsCap": true } option:
js
/*eslint new-cap: ["error", { "newIsCap": true }]*/
var friend = new person();Examples of correct code for this rule with the default { "newIsCap": true } option:
js
/*eslint new-cap: ["error", { "newIsCap": true }]*/
var friend = new Person();Examples of correct code for this rule with the { "newIsCap": false } option:
js
/*eslint new-cap: ["error", { "newIsCap": false }]*/
var friend = new person();Examples of incorrect code for this rule with the default { "capIsNew": true } option:
js
/*eslint new-cap: ["error", { "capIsNew": true }]*/
var colleague = Person();Examples of correct code for this rule with the default { "capIsNew": true } option:
js
/*eslint new-cap: ["error", { "capIsNew": true }]*/
var colleague = new Person();Examples of correct code for this rule with the { "capIsNew": false } option:
js
/*eslint new-cap: ["error", { "capIsNew": false }]*/
var colleague = Person();Examples of additional correct code for this rule with the { "newIsCapExceptions": ["events"] } option:
js
/*eslint new-cap: ["error", { "newIsCapExceptions": ["events"] }]*/
var events = require("events");
var emitter = new events();Examples of additional correct code for this rule with the { "newIsCapExceptionPattern": "^person\\.." } option:
js
/*eslint new-cap: ["error", { "newIsCapExceptionPattern": "^person\\.." }]*/
var friend = new person.acquaintance();
var bestFriend = new person.friend();Examples of additional correct code for this rule with the { "newIsCapExceptionPattern": "\\.bar$" } option:
js
/*eslint new-cap: ["error", { "newIsCapExceptionPattern": "\\.bar$" }]*/
var friend = new person.bar();Examples of additional correct code for this rule with the { "capIsNewExceptions": ["Person"] } option:
js
/*eslint new-cap: ["error", { "capIsNewExceptions": ["Person"] }]*/
function foo(arg) {
return Person(arg);
}Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "^person\\.." } option:
js
/*eslint new-cap: ["error", { "capIsNewExceptionPattern": "^person\\.." }]*/
var friend = person.Acquaintance();
var bestFriend = person.Friend();Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "\\.Bar$" } option:
js
/*eslint new-cap: ["error", { "capIsNewExceptionPattern": "\\.Bar$" }]*/
foo.Bar();Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "^Foo" } option:
js
/*eslint new-cap: ["error", { "capIsNewExceptionPattern": "^Foo" }]*/
var x = Foo(42);
var y = Foobar(42);
var z = Foo.Bar(42);properties
Examples of incorrect code for this rule with the default { "properties": true } option:
js
/*eslint new-cap: ["error", { "properties": true }]*/
var friend = new person.acquaintance();Examples of correct code for this rule with the default { "properties": true } option:
js
/*eslint new-cap: ["error", { "properties": true }]*/
var friend = new person.Acquaintance();Examples of correct code for this rule with the { "properties": false } option:
js
/*eslint new-cap: ["error", { "properties": false }]*/
var friend = new person.acquaintance();Examples of incorrect code for this rule with the default { "newIsCap": true } option:
js
/*eslint new-cap: ["error", { "newIsCap": true }]*/
var friend = new person();Examples of correct code for this rule with the default { "newIsCap": true } option:
js
/*eslint new-cap: ["error", { "newIsCap": true }]*/
var friend = new Person();Examples of correct code for this rule with the { "newIsCap": false } option:
js
/*eslint new-cap: ["error", { "newIsCap": false }]*/
var friend = new person();Examples of incorrect code for this rule with the default { "capIsNew": true } option:
js
/*eslint new-cap: ["error", { "capIsNew": true }]*/
var colleague = Person();Examples of correct code for this rule with the default { "capIsNew": true } option:
js
/*eslint new-cap: ["error", { "capIsNew": true }]*/
var colleague = new Person();Examples of correct code for this rule with the { "capIsNew": false } option:
js
/*eslint new-cap: ["error", { "capIsNew": false }]*/
var colleague = Person();Examples of additional correct code for this rule with the { "newIsCapExceptions": ["events"] } option:
js
/*eslint new-cap: ["error", { "newIsCapExceptions": ["events"] }]*/
var events = require("events");
var emitter = new events();Examples of additional correct code for this rule with the { "newIsCapExceptionPattern": "^person\\.." } option:
js
/*eslint new-cap: ["error", { "newIsCapExceptionPattern": "^person\\.." }]*/
var friend = new person.acquaintance();
var bestFriend = new person.friend();Examples of additional correct code for this rule with the { "newIsCapExceptionPattern": "\\.bar$" } option:
js
/*eslint new-cap: ["error", { "newIsCapExceptionPattern": "\\.bar$" }]*/
var friend = new person.bar();Examples of additional correct code for this rule with the { "capIsNewExceptions": ["Person"] } option:
::: correct
js
/*eslint new-cap: ["error", { "capIsNewExceptions": ["Person"] }]*/
function foo(arg) {
return Person(arg);
}Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "^person\\.." } option:
js
/*eslint new-cap: ["error", { "capIsNewExceptionPattern": "^person\\.." }]*/
var friend = person.Acquaintance();
var bestFriend = person.Friend();Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "\\.Bar$" } option:
js
/*eslint new-cap: ["error", { "capIsNewExceptionPattern": "\\.Bar$" }]*/
foo.Bar();Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "^Foo" } option:
js
/*eslint new-cap: ["error", { "capIsNewExceptionPattern": "^Foo" }]*/
var x = Foo(42);
var y = Foobar(42);
var z = Foo.Bar(42);Examples of incorrect code for this rule with the default { "properties": true } option:
js
/*eslint new-cap: ["error", { "properties": true }]*/
var friend = new person.acquaintance();Examples of correct code for this rule with the default { "properties": true } option:
js
/*eslint new-cap: ["error", { "properties": true }]*/
var friend = new person.Acquaintance();Examples of correct code for this rule with the { "properties": false } option:
js
/*eslint new-cap: ["error", { "properties": false }]*/
var friend = new person.acquaintance();Configuration
This rule accepts a configuration object with the following properties:
capIsNew
type: boolean
default: true
true to require that all functions with names starting with an uppercase letter to be called with new.
capIsNewExceptionPattern
type: [ string, null ]
A regex pattern to match exceptions for functions with names starting with an uppercase letter.
capIsNewExceptions
type: string[]
default: []
Exceptions to ignore for functions with names starting with an uppercase letter.
newIsCap
type: boolean
default: true
true to require that all constructor names start with an uppercase letter, e.g. new Person().
newIsCapExceptionPattern
type: [ string, null ]
A regex pattern to match exceptions for constructor names starting with an uppercase letter.
newIsCapExceptions
type: string[]
default: ["Array", "Boolean", "Date", "Error", "Function", "Number", "Object", "RegExp", "String", "Symbol", "BigInt"]
Exceptions to ignore for constructor names starting with an uppercase letter.
properties
type: boolean
default: true
true to require capitalization for object properties (e.g., new obj.Method()).
How to use
To enable this rule in the CLI or using the config file, you can use:
bash
oxlint --deny new-capjson
{
"rules": {
"new-cap": "error"
}
}