Why can't I call an array method on a function's arguments?

Question

I have a function that can accept any number of arguments...

const getSearchFields = () => {        
    const joined = arguments.join(\'/\'); 
};
         


        

Answer 1

The problem is that arguments is an array-like object (it has numbered properties mapping to the passed arguments and a length property), but it actually isn't an array.

In the old days, you could make it into a real array with Array.prototype.slice.call(arguments) (among other methods).

However, we're a bit luckier these days (depending on your supported platforms of course). You can either do...

const getSearchFields = (...args) => {
   Array.isArray(args); // true
};

...or...

const getSearchFields = () => {
   const args = Array.from(arguments);
   Array.isArray(args); // true
};

The first example is preferred because...

• you get the normal array creation without an extra step in the function body
• arguments is a magic variable (it's not defined explicitly, so where did it come from?)
• it has funny behaviour with named args in non-strict mode (note that you should always 'use strict')
• the first example has much clearer intent (the creation of an array of all arguments is explcit).

Answer 2

You have to make assignment to a variable for it to work.

var a = ['Wind', 'Rain', 'Fire'];

var myVar1 = a.join();

var myVar2 = a.join(', ');

var myVar3 = a.join(' + ');

var myVar4 = a.join('');