What is the difference in methods to call upon a JSON object by key name?

Question

I tried my best to think of a name of this question, I'll change it if I can get some terminology to help

Best way I can describe it is this

obj1 = {
    "a":{"var":1},
    "b":{"var":2},
    "c":{"var":3}
}

// What's the difference between
resulta = obj1['a']['var']

// and...
resultb = obj1.a.var

So, what's the difference between using [''] and . ?

I realize you can only use . to run functions, but is that the only difference?

Is one method faster than the other? (even a little?)

You can call functions using array accessors: `var obj = {someFunc: function() { console.log('hi'); }};` - `obj['someFunc']();` works just fine. — dontGoPlastic, May 14 '12 at 22:17

score 1 · Accepted Answer · answered May 14 '12 at 22:05

1

The first method with square brackets is handy if you're dynamically building your object's property keys:

var myDynamicKey = "a";
var result = obj[myDynamicKey]["var"];
var result = obj["someOtherKey_" + myDynamicKey]["var"];

The second one is definitely preferred if you know what the properties are in advance.

Note you can mix and match them:

var result = obj[myDynamicKey].var;

I'd be willing to bet that accessing properties using dot notation is faster, but I have no actual data to support that.

answered May 14 '12 at 22:05

dontGoPlastic

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Since `var` is a reserved keyword in JavaScript, the last bit will probably not work with every interpreter. – Niko May 14 '12 at 22:09
I was just trying to mimic the OP's object structure. – dontGoPlastic May 14 '12 at 22:10

score 1 · Answer 2 · answered May 14 '12 at 22:06

If you use the [''] then you can pass the name of the key in as a dynamic variable... which can change at run time. If you use the .key.key method, then you have to know at build time what the keys are.

Example:

var keys = ['name','phone','email'];
var object = {"name": ,"phone": , "email"};
function updateKeys(name,phone,email){
    for(var i = 0; i < keys; i++){
        object[keys[i]] = arguments[i]
    }
}

vs

function updateKeys(name, phone, email){
    object.name = name; 
    object.phone = phone;
    object.email = email
}

The [''] way is much more flexible and allows for more code resuse. Most libraries will use this, or some homegrown replacement of the [''] way of doing things.

score 0 · Answer 3 · answered May 14 '12 at 22:05

There are various websites with information about this.

http://www.quirksmode.org/js/associative.html
or
http://www.hunlock.com/blogs/Mastering_JSON_(_JavaScript_Object_Notation_)
or
http://www.mongodb.org/display/DOCS/Dot+Notation+(Reaching+into+Objects)

The first I would call dot notation which can be used to access Methods and Properties of an object, and the 2nd is Associative Array.

What is the difference in methods to call upon a JSON object by key name?

3 Answers3