Question

To clarify my title, I need a way to determine that an object is not a String, Number, Boolean, or any other predefined JavaScript object. One method that comes to mind is this:

if(!typeof myCustomObj == "string" && !typeof myCustomObj  == "number" && !typeof myCustomObj == "boolean") {

I could check to see if myCustomObj is an object, like this:

if(typeof myCustomObj == "object") {

This only works for primitive values, though, as this typeof new String("hello world") == "object") is true.

What is a reliable way to determine whether or not an object is not a predefined JavaScript object?

Answer 1

Here’s is how jQuery does it in jQuery.isPlainObject()

function (obj) {
    // Must be an Object.
    // Because of IE, we also have to check the presence of the constructor property.
    // Make sure that DOM nodes and window objects don't pass through, as well
    if (!obj || jQuery.type(obj) !== "object" || obj.nodeType || jQuery.isWindow(obj)) {
        return false;
    }

    try {
        // Not own constructor property must be Object
        if (obj.constructor && !hasOwn.call(obj, "constructor") && !hasOwn.call(obj.constructor.prototype, "isPrototypeOf")) {
            return false;
        }
    } catch(e) {
        // IE8,9 Will throw exceptions on certain host objects #9897
        return false;
    }

    // Own properties are enumerated firstly, so to speed up,
    // if last one is own, then all properties are own.
    var key;
    for (key in obj) {}

    return key === undefined || hasOwn.call(obj, key);
}