JavaScript Truthiness Quiz

I’ve got another quirky JavaScript quiz for you. This one has two different answers.

function foo(object) {
    object.bar = false;
    return object.bar && true;
}

foo(___); // Fill in an argument such that foo() returns true.

Obviously a normal object won’t do the job. Something more special is needed.

foo({bar: true});  // => false

The fact that foo() can return true could introduce a bug during refactoring: the code initially appears to be a tautology that could be reduced to a simpler return false. Since this quiz has solutions that’s obviously not true.

Had I reversed the booleans — assign bar to true and make this function return a falsy value — then almost any immutable object, such as a string, would do.

function foo2(object) {
    object.bar = true;  // inverse
    return object.bar && true;
}

foo2("baz");  // => undefined

The bar assignment would fail and attempting to access it would return undefined, which is falsy.

Answer

The two approaches are getters and property descriptors.

Getters

JavaScript directly supports getter and setter properties. Without special language support, such accessors could be accomplished with plain methods (like Java).

var lovesBlue = {
    _color: "blue",  // private

    getColor: function() {
        return this._color;
    },

    /* Only blue colors are allowed! */
    setColor: function(color) {
        if (/blue/.test(color)) {
            this._color = color;
        }
        return this._color;
    }
};

lovesBlue.getColor();              // => "blue"
lovesBlue.setColor("red");         // => "blue" (set fails)
lovesBlue.setColor("light blue");  // => "light blue"

JavaScript allows properties themselves to transparently run methods, such as to enforce invariants, even though it’s not an obvious call site. This is how many of the browser environment objects work. There’s a special syntax with get and set keywords. (Keep this mind, JSON parser writers!)

var lovesBlue = {
    _color: "blue",

    get color() {
        return this._color;
    },

    set color(color) {
        if (/blue/.test(color)) {
            this._color = color;
        }
    }
};

lovesBlue.color = "red";  // => "red", but assignment fails
lovesBlue.color;          // => "blue"

This can be used to solve the quiz,

foo({get bar() { return true; }});

Because bar is a getter with no associated setter, there’s effectively no assigning values to bar and it always evaluates to true.

Property descriptors

Object properties themselves have properties, called descriptors, governing their behavior. The accessors above are examples of the descriptors get and set. For our situation there’s a writable descriptor which determines whether or not a particular property can be assigned. If you really wanted to lock this in, there’s even a metadescriptor (a metametaproperty?), configurable, that determines whether or not a property’s descriptors, including itself, can be modified.

There’s no literal syntax for them, but these descriptors can be set with Object.defineProperty(). Conveniently, this function returns the object being modified.

foo(Object.defineProperty({}, 'bar', {value: true, writable: false}));

This creates a new object, sets bar to true, and locks that in by making the property read-only.

The fact that attempting to assign a read-only property silently fails instead of throwing an exception is probably another mistake in the language’s design. While this behavior newbie-friendly, it allows bugs to slip by undetected, only to be found much later when they’re more expensive to address. It makes JavaScript programs more brittle.

Existing objects: a third approach?

If you’re lazy and in a browser environment, you don’t even need to construct new objects to solve the problem. There are some already lying around! My favorite is HTML5’s localStorage. It stringifies all property assignments. This means that false becomes "false", which is truthy.

foo(localStorage);  // => true

This is arguably a third approach because the stringification behavior can’t be accomplished with either normal accessors or descriptors alone.

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Chris Wellons