new property descriptors syntax in javascript includes getters and setters

get-set-property-descriptors-example.js

var t = Object.create({}, {
    __percent__: {
        value: 0.01,
        writable: true
    },
    percent: {
        get: function() {
            return this.__percent__*100
        },
        set: function(v) {
            return this.__percent__= Math.max( 0, Math.min( v, 100 ) ) / 100
        }
    }
})

Example of how and why to use get/set property descriptors in JavaScript/ES5. By creating a property that uses get/set you can create a virtual property, that can implement magical behaviour. In this example percent property enables manipulating percent by assinging values between 1...100, but the value is stored on the object in a percent property using floating point number.