javascript - How does [b][b = a,0] swap between a and b? - Stack Overflow

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As gdoron pointed out,

var a = "a";
var b = "b";

a = [b][b = a,0];

Will swap a and b, and although it looks a bit of hacky, it has triggered my curiosity and I am very curious at how it works. It doesn't make any sense to me.

As gdoron pointed out,

var a = "a";
var b = "b";

a = [b][b = a,0];

Will swap a and b, and although it looks a bit of hacky, it has triggered my curiosity and I am very curious at how it works. It doesn't make any sense to me.

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Share Improve this question Follow edited May 23, 2017 at 11:48 CommunityBot 111 silver badge asked Mar 25, 2012 at 21:52 Derek 朕會功夫Derek 朕會功夫 94.4k4545 gold badges197197 silver badges253253 bronze badges 4

• 2 "a bit of hacky"??? a lot of hacky... :-) – gdoron Commented Mar 25, 2012 at 21:59
• Relevant: What's a better way to swap two argument values? – Šime Vidas Commented Mar 25, 2012 at 22:05
• Wouldn't it have made more sense to ment on godron's other answer and ask for clarification in your original question? – Felix Kling Commented Mar 25, 2012 at 22:36
• 1 @FelixKling. I suggested him to ask it in other question. ments are not the place of questions-answers. And it's not just clarification of the answer, it's totally different question (In my humble opinion!) – gdoron Commented Mar 25, 2012 at 23:00

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 9

var a = "a";
var b = "b";

a = [b][b = a, 0];

Let's break the last line into pieces:

[b]       // Puts b in an array - a safe place for the swap.
[b = a]   // Assign a in b
[b = a,0] // Assign a in b and return the later expression - 0 with the ma operator.

so finally it is a =[b][0] - the first object in the [b] array => b assigned to a

Live DEMO

read @am not I am ments in this question:
When is the ma operator useful?
It's his code...

It might help (or hinder) to think of it terms of the semantically equivalent lambda construction (here, parameter c takes the place of element 0):

a = (function(c) { b = a; return c; })(b);

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