func-names

Require or disallow named function expressions

A pattern that’s becoming more common is to give function expressions names to aid in debugging. For example:

Foo.prototype.bar = function bar() {};
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Adding the second bar in the above example is optional. If you leave off the function name then when the function throws an exception you are likely to get something similar to anonymous function in the stack trace. If you provide the optional name for a function expression then you will get the name of the function expression in the stack trace.

Rule Details

This rule can enforce or disallow the use of named function expressions.

Options

This rule has a string option:

• "always" (default) requires function expressions to have a name
• "as-needed" requires function expressions to have a name, if the name isn’t assigned automatically per the ECMAScript specification.
• "never" disallows named function expressions, except in recursive functions, where a name is needed

This rule has an object option:

• "generators": "always" | "as-needed" | "never"
  • "always" require named generators
  • "as-needed" require named generators if the name isn’t assigned automatically per the ECMAScript specification.
  • "never" disallow named generators where possible.

When a value for generators is not provided the behavior for generator functions falls back to the base option.

Please note that "always" and "as-needed" require function expressions and function declarations in export default declarations to have a name.

always

Examples of incorrect code for this rule with the default "always" option:

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function() {};

const cat = {
  meow: function() {}
}

(function() {
    // ...
}())

export default function() {}
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Examples of correct code for this rule with the default "always" option:

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function bar() {};

const cat = {
  meow() {}
}

(function bar() {
    // ...
}())

export default function foo() {}
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as-needed

ECMAScript 6 introduced a name property on all functions. The value of name is determined by evaluating the code around the function to see if a name can be inferred. For example, a function assigned to a variable will automatically have a name property equal to the name of the variable. The value of name is then used in stack traces for easier debugging.

Examples of incorrect code for this rule with the "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

Foo.prototype.bar = function() {};

(function() {
    // ...
}())

export default function() {}
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Examples of correct code for this rule with the "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

var bar = function() {};

const cat = {
  meow: function() {}
}

class C {
    #bar = function() {};
    baz = function() {};
}

quux ??= function() {};

(function bar() {
    // ...
}())

export default function foo() {}
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never

Examples of incorrect code for this rule with the "never" option:

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function bar() {};

(function bar() {
    // ...
}())
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Examples of correct code for this rule with the "never" option:

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function() {};

(function() {
    // ...
}())
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generators

Examples of incorrect code for this rule with the "always", { "generators": "as-needed" } options:

/*eslint func-names: ["error", "always", { "generators": "as-needed" }]*/

(function*() {
    // ...
}())
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Examples of correct code for this rule with the "always", { "generators": "as-needed" } options:

/*eslint func-names: ["error", "always", { "generators": "as-needed" }]*/

var foo = function*() {};
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Examples of incorrect code for this rule with the "always", { "generators": "never" } options:

/*eslint func-names: ["error", "always", { "generators": "never" }]*/

var foo = bar(function *baz() {});
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Examples of correct code for this rule with the "always", { "generators": "never" } options:

/*eslint func-names: ["error", "always", { "generators": "never" }]*/

var foo = bar(function *() {});
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Examples of incorrect code for this rule with the "as-needed", { "generators": "never" } options:

/*eslint func-names: ["error", "as-needed", { "generators": "never" }]*/

var foo = bar(function *baz() {});
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Examples of correct code for this rule with the "as-needed", { "generators": "never" } options:

/*eslint func-names: ["error", "as-needed", { "generators": "never" }]*/

var foo = bar(function *() {});
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Examples of incorrect code for this rule with the "never", { "generators": "always" } options:

/*eslint func-names: ["error", "never", { "generators": "always" }]*/

var foo = bar(function *() {});
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Examples of correct code for this rule with the "never", { "generators": "always" } options:

/*eslint func-names: ["error", "never", { "generators": "always" }]*/

var foo = bar(function *baz() {});
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Compatibility

• JSCS: requireAnonymousFunctions
• JSCS: disallowAnonymousFunctions

Version

This rule was introduced in ESLint v0.4.0.

Further Reading

Functions Explained - Mark Daggett’s Blog
web.archive.org

The names of functions in ES6
2ality.com

Resources

• Rule source
• Tests source