Function.prototype.call()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The call()
method of Function
instances calls this function with a given this
value and arguments provided individually.
Try it
Syntax
call(thisArg)
call(thisArg, arg1)
call(thisArg, arg1, arg2)
call(thisArg, arg1, arg2, /* …, */ argN)
Parameters
thisArg
-
The value to use as
this
when callingfunc
. If the function is not in strict mode,null
andundefined
will be replaced with the global object, and primitive values will be converted to objects. arg1
, …,argN
Optional-
Arguments for the function.
Return value
The result of calling the function with the specified this
value and arguments.
Description
Note: This function is almost identical to apply()
, except that the function arguments are passed to call()
individually as a list, while for apply()
they are combined in one object, typically an array — for example, func.call(this, "eat", "bananas")
vs. func.apply(this, ["eat", "bananas"])
.
Normally, when calling a function, the value of this
inside the function is the object that the function was accessed on. With call()
, you can assign an arbitrary value as this
when calling an existing function, without first attaching the function to the object as a property. This allows you to use methods of one object as generic utility functions.
Warning: Do not use call()
to chain constructors (for example, to implement inheritance). This invokes the constructor function as a plain function, which means new.target
is undefined
, and classes throw an error because they can't be called without new
. Use Reflect.construct()
or extends
instead.
Examples
Using call() to invoke a function and specifying the this value
In the example below, when we call greet
, the value of this
will be bound to object obj
, even when greet
is not a method of obj
.
function greet() {
console.log(this.animal, "typically sleep between", this.sleepDuration);
}
const obj = {
animal: "cats",
sleepDuration: "12 and 16 hours",
};
greet.call(obj); // cats typically sleep between 12 and 16 hours
Using call() to invoke a function without specifying the first argument
If the first thisArg
parameter is omitted, it defaults to undefined
. In non-strict mode, the this
value is then substituted with globalThis
(which is akin to the global object).
globalThis.globProp = "Wisen";
function display() {
console.log(`globProp value is ${this.globProp}`);
}
display.call(); // Logs "globProp value is Wisen"
In strict mode, the value of this
is not substituted, so it stays as undefined
.
"use strict";
globalThis.globProp = "Wisen";
function display() {
console.log(`globProp value is ${this.globProp}`);
}
display.call(); // throws TypeError: Cannot read the property of 'globProp' of undefined
Transforming methods to utility functions
call()
is almost equivalent to a normal function call, except that this
is passed as a normal parameter instead of as the value that the function was accessed on. This is similar to how general-purpose utility functions work: instead of calling array.map(callback)
, you use map(array, callback)
, which allows you to use map
with array-like objects that are not arrays (for example, arguments
) without mutating Object.prototype
.
Take Array.prototype.slice()
, for example, which you want to use for converting an array-like object to a real array. You could create a shortcut like this:
const slice = Array.prototype.slice;
// ...
slice.call(arguments);
Note that you can't save slice.call
and call it as a plain function, because the call()
method also reads its this
value, which is the function it should call. In this case, you can use bind()
to bind the value of this
for call()
. In the following piece of code, slice()
is a bound version of Function.prototype.call()
, with the this
value bound to Array.prototype.slice()
. This means that additional call()
calls can be eliminated:
// Same as "slice" in the previous example
const unboundSlice = Array.prototype.slice;
const slice = Function.prototype.call.bind(unboundSlice);
// ...
slice(arguments);
Specifications
Specification |
---|
ECMAScript Language Specification # sec-function.prototype.call |
Browser compatibility
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