112 
var a,b,c;
var arr = [1,2,3];
[a,b,c] = arr;

this code works perfectly in Firefox resulting a=1, b=2 and c=3,
but it doesn't work in Chrome. Is it a Chrome bug or
it is not valid javascript code? (I failed to find it in javascript references)

How can I modify this code to make it suitable for Chrome, with minimum damage to it?
(I don't really like to write a = arr[0]; b = arr[1]... or the same with arr.shift() all the time)

P.S. this is just an example code, in real code
I get the arr array from somewhere outside my code

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  • 1
    What happens in Chrome? What error messages do you get? Commented Aug 8, 2011 at 13:46
  • it gives me the following: ReferenceError arguments: Array[0] message: "—" stack: "—" type: "invalid_lhs_in_assignment" proto: Error Commented Aug 8, 2011 at 13:47
  • 1
    FWIW, jslint.com says it's fine (after fixing some whitespace; though I don't know what it would evaluate to), but jshint.com says it's a bad assignment. Commented Aug 8, 2011 at 13:47
  • Interesting! This is some weird construct, I'm surprised it works on most browsers. Curious to know it this is even valid JS (apparently valid but could be due to engine's implementation). Commented Aug 8, 2011 at 13:52
  • 2
    As to your actual question about what code you could use in Chrome, here's a previous discussion of that: stackoverflow.com/questions/204444/…. Commented Aug 8, 2011 at 14:09

2 Answers 2

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129

This is a new feature of JavaScript 1.7 called Destructuring assignment:

Destructuring assignment makes it possible to extract data from arrays or objects using a syntax that mirrors the construction of array and object literals.

The object and array literal expressions provide an easy way to create ad-hoc packages of data. Once you've created these packages of data, you can use them any way you want to. You can even return them from functions.

One particularly useful thing you can do with destructuring assignment is to read an entire structure in a single statement, although there are a number of interesting things you can do with them, as shown in the section full of examples that follows.

You can use destructuring assignment, for example, to swap values:

var a = 1;
var b = 3;
[a, b] = [b, a];

This capability is similar to features present in languages such as Perl and Python.

Unfortunately, according to this table of versions, JavaScript 1.7 has not been implemented in Chrome. But it should be there in:

  • FireFox 2.0+
  • IE 9
  • Opera 11.50.

Try it for yourself in this jsfiddle: http://jsfiddle.net/uBReg/

I tested this on Chrome (failed), IE 8 (failed), and FireFox 5 (which worked, per the wiki table).

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9 Comments

Destructuring is (likely) coming as an official language feature in the next version of ECMAScript, so Google Chrome will certainly get there assuming the proposal sticks.
That would be great; there are some exciting features in JavaScript 1.7+ ... its unfortunate that they cannot be used for "real-world" applications at the moment due to these browser incompatibilities.
Probably not. It will be slightly more efficient to say a = arr[0]; b = arr[1]; as opposed to a = arr.shift(); because shift alters the array. But either is fine - I would just pick the style you prefer.
The table of Javascript versions suggests that this works on IE9, but, for me, it throws an Error: 'Invalid Left-Hand Assignment.'
Declation and assignment at once: let [a, b] = [1, 2];
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13

It is possible only for Javascript 1.7 as already answered by @Justin. Here is a trial to simulate it in the widespread browsers:

function assign(arr, vars) {
    var x = {};
    var num = Math.min(arr.length, vars.length);
    for (var i = 0; i < num; ++i) {
        x[vars[i]] = arr[i];
    }
    return x;
}
var arr = [1, 2, 3];
var x = assign(arr, ['a', 'b', 'c']);
var z = x.a + x.b + x.c;  // z == 6

I don't know how useful it is.

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