Function.name

A Function object's read-only name property indicates the function's name as specified when it was created, or it may be rather anonymous or ''(an empty string) for functions created anonymously.

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JavaScript compressors and minifiers

Source code such as:

function Foo() {};
let foo = new Foo();

if (foo.constructor.name === 'Foo') {
  console.log("'foo' is an instance of 'Foo'");
} else {
  console.log('Oops!');
}

may be compressed to:

function a() {};
let b = new a();
if (b.constructor.name === 'Foo') {
  console.log("'foo' is an instance of 'Foo'");
} else {
  console.log('Oops!');
}

In the uncompressed version, the program runs into the truthy-branch and logs "'foo' is an instance of 'Foo'". Whereas, in the compressed version it behaves differently, and runs into the else-branch. If you rely on Function.name, like in the example above, make sure your build pipeline doesn't change function names, or don't assume a function to have a particular name.

Examples

Function statement name

The name property returns the name of a function statement.

function doSomething() {}
doSomething.name; // "doSomething"

Function constructor name

Functions created with the syntax new Function(...) or just Function(...) create Function objects and their name is "anonymous".

(new Function).name; // "anonymous"

Anonymous function expression

Anonymous function expressions that were created using the keyword function or arrow functions would have ""(an empty string) as their name.

(function() {}).name; // ""
(() => {}).name; // ""

Inferred function names

Variables and methods can infer the name of an anonymous function from its syntactic position (new in ECMAScript 2015).

let f = function() {};
let object = {
  someMethod: function() {}
};

console.log(f.name); // "f"
console.log(object.someMethod.name); // "someMethod"

You can define a function with a name in a function expression:

let object = {
  someMethod: function object_someMethod() {}
};
console.log(object.someMethod.name); // logs "object_someMethod"

try { object_someMethod } catch(e) { console.log(e); }
// ReferenceError: object_someMethod is not defined

The name property is read-only and cannot be changed by the assigment operator:

 let object = {
  // anonymous
  someMethod: function() {}
};

object.someMethod.name = 'otherMethod';
console.log(object.someMethod.name); // someMethod

To change it, use Object.defineProperty().

Shorthand method names

var o = {
  foo(){}
};
o.foo.name; // "foo";

Bound function names

Function.bind() produces a function whose name is "bound " plus the function name.

function foo() {};
foo.bind({}).name; // "bound foo"

Function names for getters and setters

When using get and set accessor properties, "get" or "set" will appear in the function name.

let o = {
  get foo(){},
  set foo(x){}
};

var descriptor = Object.getOwnPropertyDescriptor(o, "foo");
descriptor.get.name; // "get foo"
descriptor.set.name; // "set foo";

Function names in classes

You can use obj.constructor.name to check the "class" of an object (but be sure to read the warnings below):

function Foo() {}  // ES2015 Syntax: class Foo {}

var fooInstance = new Foo();
console.log(fooInstance.constructor.name); // logs "Foo"

Therefore we can't obtain the class name for virtually any class with a static method property name():

class Foo {
  constructor() {}
  static name() {}
}

With a static name() method Foo.name no longer holds the actual class name but a reference to the name() function object. Above class definition in ES2015 syntax will behave in Chrome or Firefox similar to the following snippet in ES5 syntax:

function Foo() {}
Object.defineProperty(Foo, 'name', { writable: true });
Foo.name = function() {};

Trying to obtain the class of fooInstance via fooInstance.constructor.name won't give us the class name at all but a reference to the static class method. Example:

let fooInstance = new Foo();
console.log(fooInstance.constructor.name); // logs function name()

You may also see from the ES5 syntax example that in Chrome or Firefox our static definition of Foo.name becomes writable. The built-in definition in the absence of a custom static definition is read-only:

Foo.name = 'Hello';
console.log(Foo.name); // logs "Hello" if class Foo has a static name() property but "Foo" if not.

Therefore you may not rely on the built-in Function.name property to always hold a class's name.

Symbols as function names

If a Symbol is used a function name and the symbol has a description, the method's name is the description in square brackets.

let sym1 = Symbol("foo");
let sym2 = Symbol();
let o = {
  [sym1]: function(){},
  [sym2]: function(){}
};

o[sym1].name; // "[foo]"
o[sym2].name; // ""

Specifications

Browser compatibility