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up vote 95 down vote favorite
21

I want to add new property to 'myObj', name it 'string1' and give it a value of 'string2', but when I do it it returns 'undefined:

var myObj = new Object;
var a = 'string1';
var b = 'string2';
myObj.a = b;

alert(myObj.string1); //returns 'undefined'
alert(myObj.a); //returns 'string2'

In other words: How to create an object property and give it the name stored in the variable but not the name of the variable itself? Thanks.

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7 Answers 7

up vote 136 down vote accepted

There's the dot notation and the array notation

myObj[a] = b;
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4  
They're not really "equivalent"; myObj.a = b means something different to myObj[a] = b (unless a == 'a') but whatever... this is what you want. –  Dominic Cooney Feb 11 '10 at 2:44
10  
What I meant is that they're equivalent ways to set a property. Obviously they behave differently (hence the purpose of posting this answer) - but they have the same result. –  philfreo Feb 11 '10 at 2:46
2  
philfreo: except that they don't have the same result, which is the basis for this question. –  bukzor Aug 24 '11 at 22:00
10  
I was just saying there are two ways to set an object, one for when the key is hard coded and one when it's variable. I meant the outcome of the object is equivalent regardless if you use myObj['foo'] = 'bar' or myObj.foo = 'bar' –  philfreo Aug 25 '11 at 3:46
up vote 28 down vote

Dot notation and the properties are equivalent. So you would accomplish like so:

var myObj = new Object;
var a = 'string1';
myObj[a] = 'whatever';
alert(myObj.string1)

(alerts "whatever")

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+1 Missed it by 2 minutes! Considering how detailed you were in your answer, I imagine you may be thinking "I could've shaved off two minutes in my explanation and those 100 extra upvotes would be mine!" :-) –  Jeremy Moritz Aug 1 at 16:56
up vote 4 down vote

Ecu, if you do myObj.a, then it looks for the property named a of myObj. If you do myObj[a] =b then it looks for the a.valueOf() property of myObj.

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up vote 2 down vote

You could just use this:

function createObject(propName, propValue){
    this[propName] = propValue;
}
var myObj1 = new createObject('string1','string2');

Anything you pass as the first parameter will be the property name, and the second parameter is the property value.

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up vote 1 down vote

ES6 introduces computed property names, which allow you to do

var myObj = {[a]: b};

Note browser support is currently negligible.

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up vote 0 down vote

Oneliner:

obj = (function(attr, val){ var a = {}; a[attr]=val; return a; })('hash', 5);

Or:

attr = 'hash';
val = 5;
var obj = (obj={}, obj[attr]=val, obj);

Anything more shorter?

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1  
I'm not sure if you can count that as a one-liner, as the function inside has three statements, not one (which is usually the requirement of being a one-liner). –  Andrew Larsson Apr 27 at 2:50
    
en.wikipedia.org/wiki/One-liner_program –  user2846569 Apr 27 at 12:39
up vote 0 down vote

You cannot use a variable to access a property via dot notation, instead use the array notation.

var obj= {
     'name' : 'jroi'
};
var a = 'name';
alert(obj.a); //will not work
alert(obj[a]); //should work and alert jroi'
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