Unfamiliar Javascript Syntax

Question

Long and short of the story is, whilst reading John Resig's blog (specifically http://ejohn.org/blog/javascript-trie-performance-analysis/) I came across a line which makes absolutely no sense to me whatsoever. Essentially it boils down to

object = object[key] = something;

(this can be found in the first code block of the article I've linked.)

This has proven rather difficult to google, so if anyone can offer some insight / a good online resource for me to learn for myself, I'd much appreciate it.

To claify on a single point, the object in the multiple assignment is the same ie.

A = A[key] = something;

Perhaps my question should be, what's the point in doing this?

[http://stackoverflow.com/a/1758912/1225328](http://stackoverflow.com/a/1758912/1225328) — sp00m, Sep 24 '12 at 09:40

score 0 · Accepted Answer · answered Sep 24 '12 at 09:36

0

The line assigns something to object[key] and object. You can read it like so:

object[key] = something;
object = object[key];

answered Sep 24 '12 at 09:36

zeebonk

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marked as answer as 1) was first 2) clear and concise! Thanks. – Ben Emery Sep 24 '12 at 09:52

mplungjan · Answer 2 · 2012-09-24T09:42:54.707

It is called Bracket notation

Properties of JavaScript objects can also be accessed or set using a bracket notation. Objects are sometimes called associative arrays, since each property is associated with a string value that can be used to access it. So, for example, you could access the properties of the myCar object as follows:

myCar["make"] = "Ford";
myCar["model"] = "Mustang";
myCar["year"] = 1969;

UPDATE Ah, I missed the two equals. - multiple assignment as mentioned in another answer

NOTE: It is unlikely that you would want

object=object[somekey]=something

since it would overwrite the object you just used in the middle but rather

objectA=objectB[somekey]=something

where objectA would be the same type as objectB[somekey] and something

score 0 · Answer 3 · answered Sep 24 '12 at 09:37

0

It's called multiple assignment.

Let's make an example:

a = b = 5;

After this instruction the value of both a and b is 5.

It's like doing:

a = 5;
b = 5;

answered Sep 24 '12 at 09:37

napolux

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You're wrong, `a = b = 5` is like `b = 5; a = 5`, not `a = 5; b = 5`. – sp00m Sep 24 '12 at 09:41
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Right, or better... b = 5; a = b; – napolux Sep 24 '12 at 09:42
I have edited my question ever so slightly, to account that the object for each assignment is the same. – Ben Emery Sep 24 '12 at 09:45

score 0 · Answer 4 · answered Sep 24 '12 at 09:41

0

It's a combination of multiple assignment and bracket notation.

You could also read it like this:

// Key is a string value!
var key = 'foobar';
object[key] = something;    
object = something;


// Or, if key is known
object.foobar = something;
object = something;

answered Sep 24 '12 at 09:41

SvenFinke

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Great you can take two answers and join them into one - well done :| – mplungjan Sep 24 '12 at 09:44

Unfamiliar Javascript Syntax

4 Answers4