Extending observable functionality

blocks.observable.extend() method could be used to extend a particular observable functionality. Great example is value formatting. Let's build value formatter.

Before using extend() you will need to implement the formatting logic. After that the formatter could be used for any observable:

// Creating an extender. It should be added as a property on the blocks.observable object
blocks.observable.formatter = function (formatCallback) {
    // The idea behind the formatter is to create an additional property called displayValue
    // which could be used in data-queries to show the formatter value and in the same time
    // use the original observable to work with the raw integer value
    // <span>{{goldPrice.displayValue}}</span>

    // creating a displayValue property on the observable because this points to the observable
    // creating a dependency observable so we could control value assignments
    this.displayValue = blocks.observable({

        // sets the value by calling the format callback and assigning its result
        set: function (value) {
            // this points to the displayValue observable
            this._value = formatCallback(value);

        // returns the value
        get: function () {
            // this points to the displayValue observable
            return this._value;

    this.on('change', function () {


    // it is possible to return an observable here which will return the observable when calling this extender
    // this is not necessary here because by default if you do not return observable the extender will return itself
    // so writing return this; will have the same effect as leaving it without the return

// initializing a formatter, providing the format callback parameter
// the code alerts formatted because the value is formatted initially
// Note: every parameter after the first is passed to the extender function
var goldPrice = blocks.observable(3).extend('formatter', function (value) {
    // a simple example that parses the value and appends to zeros to the end
    return parseInt(value, 10) + '.00';

// alerts 3

// alerts 3.00

// alerts formatted

// alerts 41

// alerts 41.00