Is this assignment correct?

Question

In some js library I found this code snippet:

var start, end, sel, scrollPos, subst;

[start, end, scrollPos, sel] = getSelection();

Imo this is not valid assignment but code seems to be working. Can you help?

EDIT:

longer version:

...
var start, end, sel, scrollPos, subst;
        if (typeof(document["selection"]) != "undefined") {
            sel = document.selection.createRange().text;
        } else if (typeof(textarea["setSelectionRange"]) != "undefined") {
            [start, end, scrollPos, sel] = getSelection();
        }
...

is this just an example or is it exactly the code you've seen? — Fabrizio Calderan, Mar 04 '14 at 15:08
It is almoast exactly what I have here. It is part of some markdown toolbar extension. — Entity Black, Mar 04 '14 at 15:09
This looks very similar to Pythons variable unpacking mechanism. The idea is that a list or tuple is unpacked linearly on the variables. This is often used to allow functions to return multiple values. I haven't heard that it is also available for JS. — Constantinius, Mar 04 '14 at 15:09
I get `Uncaught ReferenceError: Invalid left-hand side in assignment` — j08691, Mar 04 '14 at 15:10
It doesn't seem to be working here: `Uncaught ReferenceError: Invalid left-hand side in assignment`. More context is needed. Possibly what you are looking at isn't JS. — Quentin, Mar 04 '14 at 15:10
Is this a firefox extension or something ? Looks to me like browser dependent code. — axelduch, Mar 04 '14 at 15:10
In Chromium this seems to work: `start, end, scrollPos, sel = getSelection()` — Constantinius, Mar 04 '14 at 15:12
@j08691 I have same error in my netbeans ide. But Im not sure it is trustworthy in this case... — Entity Black, Mar 04 '14 at 15:13
@Constantinius this is not the same as `[start, end, scrollPos, sel] = getSelection()` you're only assigning `getSelection()` to `sel` — axelduch, Mar 04 '14 at 15:13
@Bergi Your overview is always stunning. I couldnt find it there ;) Perhaps wrong keywords :) — Entity Black, Mar 04 '14 at 15:24
@Windkiller: Nah, I was searching for "destructuring assignment" to find a matching duplicate - you of course couldn't know that term… — Bergi, Mar 04 '14 at 15:30

score 5 · Accepted Answer · answered Mar 04 '14 at 15:13

This appears to be a JS 1.7 feature 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.

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

...farther down the page:

You can use destructuring assignment, for example, to swap values:
var a = 1;
var b = 3;
[a, b] = [b, a];

As far as using the feature, it appears to only be supported by Mozilla Firefox, which means it's perfectly acceptable to use it so long as you're only supporting that browser. Browser plugins, for example, can be written to take advantage of this feature.

Is this assignment correct?

1 Answers1

Linked