Question

Can you guys help me make a shorthand function determining whether an object property exists? In 99% of the cases I want to use it to check whether the returned json object contains the specified property or not. Note that there is no guarantee that any of the parent properties or even the json object itself must be defined.

I was thinking of something in this manner:

function propertyExists(<property>) {
    // property is something like data.property.property
    return typeof(data) !== "undefined" && typeof(data.property) !== "undefined" && typeof(data.property.property) !== "undefined";
}

I don’t know how to write it in a dynamic manner to check all the parent properties. Also the in-parameter should be just a reference to the “data.property.property” and not a string, so I don’t know how to find the parent properties within that either.

Answer 1

Here’s a function I have lying around from a project that tells you if a property is defined (including all of its parent properties, if any):

function isDefined(target, path) {
    if (typeof target != 'object' || target == null) {
        return false;
    }

    var parts = path.split('.');

    while(parts.length) {
        var branch = parts.shift();
        if (!(branch in target)) {
            return false;
        }

        target = target[branch];
    }

    return true;
}

It’s supposed to be used like this:

var data = { foo: { bar: 42 } };
isDefined(data, "foo"); // true
isDefined(data, "foo.bar"); // true
isDefined(data, "notfoo"); // false
isDefined(data, "foo.baz"); // false

You can easily tweak this to return the value itself (or null) instead of true/false.

Update: After reading the comments on the question, I googled the Javascript in operator and replaced the typeof test with that. Now the code is written “how it was meant to be”.