Question

Due to object in javascript is associative map (HashMap in other programming languages) does next code

for (var prop in object) {
    if (prop === someConcreteProperty) {
        // some decision
        break;
    }
}

slower anyhow then dummy property lookup like

if (typeof object.someConcreteProperty != 'undefined') {
    // some decision
}

Edits:

I’m thinking about performance in code like:

for ( var prop in obj)
    if (obj[prop] === someAnonymousMethod) {
        // I need that property name for my need
        return obj.prop();
    }

will it be twice property lookup time like

obj.prop()

or more?

Thanks.

Answer 1

This can be tested empirically:

<script language="javascript">

alert("Initialising test object...");
var obj = new Object();
for (var i=0; i<1000000; i++) obj["prop"+i] = i;


alert("Initialised. Doing Test.");

var d1 = (new Date()).getTime();

needle = obj["prop"+(i-1)]; // last object
for (var prop in obj) {
    if (obj === needle) {
        // some decision
        break;
    }
}

var ms1 = ((new Date()).getTime()) - d1;

alert("Method 1 took "+ms1+"ms.")

var d2 = (new Date()).getTime();

if (typeof obj["prop"+(i-1)] != 'undefined') {
    // some decision
}

var ms2 = (new Date()).getTime() - d2;
alert("Method 2 took "+ms2+"ms.")

</script>

Method 1 takes MUCH longer than Method 2. This is hardly surprising since all of the computing necessary to execute Method 2 is included in Method 1 plus MUCH more.