How to efficiently check if variable is Array or Object (in NodeJS & V8)?


Is there any way to efficiently check if the variable is Object or Array, in NodeJS & V8?

I'm writing a Model for MongoDB and NodeJS, and to traverse the object tree I need to know if the object is simple (Number, String, ...) or composite (Hash, Array).

It seems that V8 has fast built-in Array.isArray, but how to check if object is an Object? I mean complex object like hash {} or instance of class, not something like new String()?

Usually it may be done as this: == "[object Object]"

or this:

object === Object(object)

But it seems that this operations aren't cheap, maybe there's some more efficient? It's ok if it's not universal and doesn't works on all engines, I need it only to work on V8.

1/12/2012 11:17:47 AM

Accepted Answer

All objects are instances of at least one class – Object – in ECMAScript. You can only differentiate between instances of built-in classes and normal objects using Object#toString. They all have the same level of complexity, for instance, whether they are created using {} or the new operator. is your best bet to differentiate between normal objects and instances of other built-in classes, as object === Object(object) doesn't work here. However, I can't see a reason why you would need to do what you're doing, so perhaps if you share the use case I can offer a little more help.

1/12/2012 11:24:46 AM

For simply checking against Object or Array without additional function call (speed).


isArray = function(a) {
    return (!!a) && (a.constructor === Array);
console.log(isArray(        )); // false
console.log(isArray(    null)); // false
console.log(isArray(    true)); // false
console.log(isArray(       1)); // false
console.log(isArray(   'str')); // false
console.log(isArray(      {})); // false
console.log(isArray(new Date)); // false
console.log(isArray(      [])); // true


isObject = function(a) {
    return (!!a) && (a.constructor === Object);
console.log(isObject(        )); // false
console.log(isObject(    null)); // false
console.log(isObject(    true)); // false
console.log(isObject(       1)); // false
console.log(isObject(   'str')); // false
console.log(isObject(      [])); // false
console.log(isObject(new Date)); // false
console.log(isObject(      {})); // true

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