Question

How does this code work?

if (!(args in pad)) {
    pad[args] = self.apply(obj, arguments);
}

args is array, but shouldn’t it be a string because it’s a key of JS object?
How would check work? array in object?

Full context here:

Function.prototype.memoize = function() {
    var pad  = {};
    var self = this;
    var obj  = arguments.length > 0 ? arguments[i] : null;

    var memoizedFn = function() {
        // Copy the arguments object into an array: allows it to be used as
        // a cache key.
        var args = [];
        for (var i = 0; i < arguments.length; i++) {
            args[i] = arguments[i];
        }

        // Evaluate the memoized function if it hasn't been evaluated with
        // these arguments before.
        if (!(args in pad)) {
            pad[args] = self.apply(obj, arguments);
        }

        return pad[args];
    }

    memoizedFn.unmemoize = function() {
        return self;
    }

    return memoizedFn;
}

Answer 1

args is converted to a string which for arrays means:

[1,2,3].toString() == "1,2,3"; //true

It is automatically converted to a string when used in in:

( [1,2,3] in ( {"1,2,3":""} ) ) //true

This happens because the in operator only accepts strings on the left side.

When you use pad[args], the same conversion happens again because object keys can only be strings. For example, when you are using array[1], what actually happens is array["1"] because the number is converted to a string.