What is the difference between call and apply?

Question

What is the difference between using Function.prototype.apply() and Function.prototype.call() to invoke a function?

var func = function() {   alert('hello!'); }; 

func.apply(); vs func.call();

Are there performance differences between the two aforementioned methods? When is it best to use call over apply and vice versa?

Answer 1

The difference is that apply lets you invoke the function with arguments as an array; call requires the parameters be listed explicitly. A useful mnemonic is "A for array and C for comma."

See MDN's documentation on apply and call.

Pseudo syntax:

theFunction.apply(valueForThis, arrayOfArgs)

theFunction.call(valueForThis, arg1, arg2, ...)

There is also, as of ES6, the possibility to spread the array for use with the call function, you can see the compatibilities here.

Sample code:

function theFunction(name, profession) {     console.log("My name is " + name + " and I am a " + profession +"."); } theFunction("John", "fireman"); theFunction.apply(undefined, ["Susan", "school teacher"]); theFunction.call(undefined, "Claude", "mathematician"); theFunction.call(undefined, ...["Matthew", "physicist"]); // used with the spread operator

Answer 2

K. Scott Allen has a nice writeup on the matter.

Basically, they differ on how they handle function arguments.

The apply() method is identical to call(), except apply() requires an array as the second parameter. The array represents the arguments for the target method."

So:

// assuming you have f function f(message) { ... } f.call(receiver, "test"); f.apply(receiver, ["test"]);