Function to convert an Array to an Associative array [duplicate]

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Closed 13 years ago.

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Convert flat array [k1,v1,k2,v2] to object {k1:v1,k2:v2} in JavaScript?

I want to convert an array to an associative array in JavaScript.

For example, given the following input,

 var a = ['a', 'b', 'c', 'd'];

I want to get the next associative array as output:

 {'a' : 'b', 'c' : 'd'}

How can I do that?

• James M: You mean to an Object? Do you want the array members to be the keys or values?
• David Rodrigues: Well... It's an associative array, but uses number like key. It's similar to: {1: 'a', 2: 'b', 3: 'c', 4: 'd'}
• James M: @DavidRodrigues But 0 based, not 1 based!
• Murtaza Khursheed Hussain: Actually I want above array to be converted into var a = ['a':'b', 'c':'d']
• epascarello: Edit your question with those details!

Answer 1

Using .forEach:

var a = ['a', 'b', 'c', 'd'];
var obj_a = {};
a.forEach(function(val, i) {
 if (i % 2 === 1) return; // Skip all even elements (= odd indexes)
 obj_a[val] = a[i + 1]; // Assign the next element as a value of the object,
 // using the current value as key
});
// Test output:
JSON.stringify(obj_a); // {"a":"b","c":"d"}

• gintas: Nice, I didn't know about array.forEach(). Does it work across all browsers?
• Felix Kling: @gintas: Only in those supporting ES5. But you can easily provide your own implementation.
• Murtaza Khursheed Hussain: Ohh thats great. But y are you doing JSON.stringify(obj_a); ?
• Rob W: @gintas "All modern browsers but IE." citation
• Murtaza Khursheed Hussain: @ROB W if you can give the exact implementation in pure javascript..

Answer 2

Try the following:

var obj = {};
for (var i = 0, length = a.length; i < length; i += 2) {
 obj[a[i]] = a[i+1];
}

• pimvdb: a.length would be sufficient since you're jumping in steps of 2 anyway.
• Rich O'Kelly: @pimvdb Doing so would be incorrect - consider an array of length 1...
• Rob W: @rich.okelly Then it would be {'a': undefined}, which is probably better than forgetting the key.
• Alexander Pavlov: I'd warn against presence checks like if (obj["foo"])... or even if ("foo" in obj)..., since these will also detect fields found on the obj's prototype (try if ("toString" in obj)... as an example). To be on the safe side, use obj.hasOwnProperty("foo") - this guarantees that the prototype will not be checked.
• pimvdb: But then again it doesn't really make sense to convert an array with an odd number of elements into such an object, because there aren't clear pairs in that case.

Answer 3

There is no such thing as an associative array, they're called Objects but do pretty much the same :-)

Here's how you would do the conversion

 var obj = {}; // "associative array" or Object
 var a = ['a', 'b', 'c', 'd'];
 for(index in a) {
 if (index % 2 == 0) {
 var key = a[index];
 var val = a[index+1];
 obj[key] = val;
 }
 }