Question

Is it possible to send a variable number of arguments to a JavaScript function, from an array?

var arr = ['a','b','c']

var func = function()
{
    // debug 
    alert(arguments.length);
    //
    for(arg in arguments)
        alert(arg);
}

func('a','b','c','d'); // prints 4 which is what I want, then 'a','b','c','d'
func(arr); // prints 1, then 'Array'

I’ve recently written a lot of Python and it’s a wonderful pattern to be able to accept varargs and send them. e.g.

def func(*args):
   print len(args)
   for i in args:
       print i

func('a','b','c','d'); // prints 4 which is what I want, then 'a','b','c','d'
func(*arr) // prints 4 which is what I want, then 'a','b','c','d'

Is it possible in JavaScript to send an array to be treated as the arguments array?

Answer 1

Update: Since ES6, you can simply use the spread syntax when calling the function:

func(...arr);

Since ES6 also if you expect to treat your arguments as an array, you can also use the spread syntax in the parameter list, for example:

function func(...args) {
  args.forEach(arg => console.log(arg))
}

const values = ['a', 'b', 'c']
func(...values)
func(1, 2, 3)

And you can combine it with normal parameters, for example if you want to receive the first two arguments separately and the rest as an array:

function func(first, second, ...theRest) {
  //...
}

And maybe is useful to you, that you can know how many arguments a function expects:

var test = function (one, two, three) {}; 
test.length == 3;

But anyway you can pass an arbitrary number of arguments…

The spread syntax is shorter and “sweeter” than apply and if you don’t need to set the this value in the function call, this is the way to go.

Here is an apply example, which was the former way to do it:

var arr = ['a','b','c'];

function func() {
  console.log(this); // 'test'
  console.log(arguments.length); // 3

  for(var i = 0; i < arguments.length; i++) {
    console.log(arguments[i]);
  }

};

func.apply('test', arr);

Nowadays I only recommend using apply only if you need to pass an arbitrary number of arguments from an array and set the this value. apply takes is the this value as the first arguments, which will be used on the function invocation, if we use null in non-strict code, the this keyword will refer to the Global object (window) inside func, in strict mode, when explicitly using ‘use strict’ or in ES modules, null will be used.

Also note that the arguments object is not really an Array, you can convert it by:

var argsArray = Array.prototype.slice.call(arguments);

And in ES6:

const argsArray = [...arguments] // or Array.from(arguments)

But you rarely use the arguments object directly nowadays thanks to the spread syntax.