Mapping key/value with javascript

Question

Is there any way to map keys and values in an array using Javascript? In my opinion, it is something similar to jQuery .map(), but instead of mapping only the value, also "maps" the keys.

Suppose I have the following array:

var names = [ 1, 2, 3, 4, 5 ];

And I like to use a function called numberToName() that I created and generate another array from this, and the result should be something like this:

var names = { "one": 1, "two": 2, "three": 3, "four": 4, "five": 5 };

Currently I use the following method:

var names = [ 1, 2, 3, 4, 5 ],
    names_tmp = {},
    names_i = 0,
    names_len = names.length;

for(; names_i < names_len; names_i++) {
    names_tmp[numberToName(names[names_i])] = names[names_i];
}

The question is: is there any way (preferably native) to improve this method? I could even use jQuery without problems. Perhaps a function similar to this:

var names = jQuery.mapKeys([ 1, 2, 3, 4, 5], function(k, v) {
    return { key: numberToName(v), value: v };
});

georg · Accepted Answer · 2013-07-31 22:43:41Z

5

You're looking for reduce:

var names = [1, 2, 3, 4, 5].reduce(function(obj, k) {
     return obj[numberToName(k)] = k, obj;
}, {});

return obj[numberToName(k)] = k, obj is a shorthand (and admittedly less readable) way to write assignment + return:

     obj[numberToName(k)] = k;
     return obj;

A simple forEach will do the job too:

names = {};
[1, 2, 3, 4, 5].forEach(function(k) {
   names[numberToName(k)] = k
})

You can also use jquery's $.each in the same way if you want.

edited Jul 31, 2013 at 22:43

answered Jul 31, 2013 at 21:12

georg

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10 Comments

dandavis Over a year ago

umm, no he's not. that's a bass-ackwards way of pretending reduce is map or foreach...

georg Over a year ago

@dandavis: I see you understood the question and edited your post. I think forEach is better than map here though (if you dislike reduce for some reason).

dandavis Over a year ago

kill the closure and i'll remove my answer.

georg Over a year ago

@dandavis: let the OP decide.

David Rodrigues Over a year ago

I think that .reduce() works better for me, because I need return the value as array directly to var. forEach will not do that. Right?

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dandavis · Accepted Answer · 2013-07-31 21:28:28Z

1

sure is, (as of IE9):

var nums={};
[ 1, 2, 3, 4, 5].forEach(function(v, i) {
    this[numberToName(v)]=v; 
}, nums);

nums;

EDIT: added an index to make it easier to tell what's going on.

EDIT: made it perfect by using forEach, which even though it's not as close to $'s .map(), does run about 3% faster than [].map. (thanks to thg435 for the tip!)

edited Jul 31, 2013 at 21:28

answered Jul 31, 2013 at 21:11

dandavis

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