Question

When specifying an interface, it is possible to include properties that are not methods. We could have defined minHeight and minWidth to simply hold numbers. But that’d have required us to compute them in the constructor, which adds code there that isn’t strictly relevant to constructing the object. It would cause problems if, for example, the inner cell of an underlined cell was changed, at which point the size of the underlined cell should also change.

This has led some people to adopt a principle of never including nonmethod properties in interfaces. Rather than directly access a simple value property, they’d use getSomething and setSomething methods to read and write the property. This approach has the downside that you will end up writing—and reading—a lot of additional methods.

Fortunately, JavaScript provides a technique that gets us the best of both worlds. We can specify properties that, from the outside, look like normal properties but secretly have methods associated with them.

var pile = {
  elements: ["eggshell", "orange peel", "worm"],
  get height() {
    return this.elements.length;
  },
  set height(value) {
    console.log("Ignoring attempt to set height to", value);
  }
};

console.log(pile.height);
// → 3
pile.height = 100;
// → Ignoring attempt to set height to 100

In object literal, the get or set notation for properties allows you to specify a function to be run when the property is read or written. You can also add such a property to an existing object, for example a prototype, using the Object.defineProperty function (which we previously used to create nonenumerable properties).

Object.defineProperty(TextCell.prototype, "heightProp", {
  get: function() { return this.text.length; }
});

var cell = new TextCell("no\nway");
console.log(cell.heightProp);
// → 2
cell.heightProp = 100;
console.log(cell.heightProp);
// → 2

You can use a similar set property, in the object passed to defineProperty , to specify a setter method. When a getter but no setter is defined, writing to the property is simply ignored.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.