Determining the type of an object in JavaScript

Question

I have a mystery object in Javascript - please could someone tell me how to initialise a similar object?

Background: I've inherited some code that gets passed a params object from Flash. I want to 'fake' the same call, which requires manually constructing the same object. But I can't work out how to take it, because I don't know what type of object it is. (It's not the content that's the problem: it's how to initialise the object.)

Please could someone tell me whether the params object following is an array, a list, a hash or something else - or at least how I can work it out for myself, given that there is no typeof in JavaScript?

function doSomething(params) {
    var txt;
    for (p in params) {
       txt = txt + "param:" + p + ", " + params[p] + "\n";
    }
    return txt;
}

Thanks!

given that there is no typeof in JavaScript? sez who? :D Try alert(typeof params); — Pekka
– Pekka, Commented Sep 13, 2010 at 18:39
JavaScript does actually have typeof - however, it returns 'object' for objects and arrays. JavaScript does not have a separate list type. — Matt Ball
– Matt Ball, Commented Sep 13, 2010 at 18:40
First of all, use console.log(params) to examine that object. — Šime Vidas
– Šime Vidas, Commented Sep 13, 2010 at 18:47

Lekensteyn · Accepted Answer · 2010-09-13 18:40:58Z

1

This will alert the type of 'params'.

function doSomething(params) {
    alert("Type of params variable: " + typeof params);
    var txt = '', p;
    for (p in params) {
       txt += "param:" + p + ", " + params[p] + "\n";
    }
    return txt;
}

If you just want to intercept the variable, and pass the param to the original function, try this:

window.originalFunction = doSomething;
window.doSomething = function(param1){
   // do something with param1
   return window.originalFunction(param1);
};

To check whether params is an array or not, try:

alert(params instanceof Array);

edited Sep 13, 2010 at 18:40

answered Sep 13, 2010 at 18:39

Lekensteyn

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Comments

Robusto · Accepted Answer · 2010-09-13 18:40:00Z

0

You can think of it as a hash. In your example, p is the property name and params[p] accesses the value of that named property.

And, as usual, Pekka is right. There most definitely is a "typeof" operator in Javascript. I don't know where you got the idea that there isn't.

answered Sep 13, 2010 at 18:40

Robusto

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1 Comment

Pekka Over a year ago

thanks. To be fair though, Javascript's typeof doesn't distinguish between arrays and objects, which I didn't know until now.

Rahul · Accepted Answer · 2010-09-13 18:40:07Z

0

From the looks of it, params looks like a dictionary

answered Sep 13, 2010 at 18:40

Rahul

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Comments

Ryan Kinal · Accepted Answer · 2010-09-13 18:48:17Z

0

There are no lists, hashes, dictionaries (etc.) in JavaScript. There are only objects and arrays (and arrays are a type of object).

Given that the code uses for... in, it looks like it would be an Object. That is, a container with named members. Beyond that, it depends on how the object was initialized.

If you would like to make a copy of the object, try using its prototype:

var copy = {};
copy.prototype = params

answered Sep 13, 2010 at 18:48

Ryan Kinal

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