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March 03, 2025 01:02
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Knockout.js Cheat Sheet

A quick reference guide to Knockout.js, covering observables, bindings, and core functionalities for building dynamic JavaScript UIs.

Core Concepts

Observables

ko.observable( value )
Creates an observable, a special JavaScript object with a value property that notifies subscribers when changed.

Example:

var myObservable = ko.observable('Initial Value');
myObservable(); // Returns 'Initial Value'
myObservable('New Value'); // Sets the value to 'New Value'

ko.observableArray( array )
Creates an observable array, tracking which objects are in the array and notifying listeners when items are added, moved, or deleted.

Example:

var myArray = ko.observableArray(['Item 1', 'Item 2']);
myArray.push('Item 3');

subscribe( callback, target, event )
Subscribes to changes in an observable. Executes the callback function whenever the observable’s value changes. target and event are optional.

Example:

myObservable.subscribe(function(newValue) {
  console.log('The new value is ' + newValue);
});

Computed Observables

ko.computed( function, target, options )
Creates a computed observable whose value is dependent on other observables and automatically updates when dependencies change.

Example:

var firstName = ko.observable('John');
var lastName = ko.observable('Doe');

var fullName = ko.computed(function() {
  return firstName() + ' ' + lastName();
});

console.log(fullName()); // Outputs: John Doe

Options for ko.computed:

  • read: Function to compute the value (default).
  • write: Function to handle writes to the computed observable.
  • pure: Indicates that the computed value is solely determined by its dependencies (default: false).
  • deferEvaluation: Boolean indicating whether the computed observable should only be evaluated when accessed (default: false).

Binding Basics

data-bind Attribute
The cornerstone of Knockout.js. Links elements in your HTML to properties in your view model.

Example:

<span data-bind="text: fullName"></span>

Common Bindings

Text and Value Bindings

text

Displays the value of an observable as text within an element.
Example:

<span data-bind="text: myObservable"></span>

value

Binds the value of a form element (e.g., input, textarea) to an observable. Supports two-way binding.
Example:

<input type="text" data-bind="value: myObservable" />

Visibility and CSS Bindings

visible

Controls the visibility of an element based on an observable value (true/false).
Example:

<div data-bind="visible: isVisible">This is visible</div>

css

Applies CSS classes to an element based on an observable or an object of observable key/value pairs.
Example:

<div data-bind="css: { highlighted: isHighlighted }"></div>

style

Applies inline styles to an element based on an observable or an object of observable key/value pairs.
Example:

<div data-bind="style: { color: textColor }"></div>

Control Flow Bindings

if

Conditionally displays an element based on an observable value. Removes the element from the DOM if the value is false.
Example:

<div data-bind="if: isLoggedIn">Welcome, user!</div>

ifnot

The opposite of if. Displays an element only if the observable value is false.
Example:

<div data-bind="ifnot: isLoggedIn">Please log in.</div>

foreach

Repeats a section of markup for each item in an observable array.
Example:

<ul data-bind="foreach: items">
  <li data-bind="text: $data"></li>
</ul>

Event Handling & Advanced Bindings

Event Bindings

click
Binds a function to the click event of an element.

Example:

<button data-bind="click: myClickHandler">Click Me</button>

var viewModel = {
  myClickHandler: function() {
    alert('Button clicked!');
  }
};

submit
Binds a function to the submit event of a form.

Example:

<form data-bind="submit: mySubmitHandler">
  ...
</form>

Template Binding

template
Renders a template with data from your view model. Templates can be inline or defined in separate script elements.

Example (Inline Template):

<div data-bind="template: { name: 'myTemplate', data: myData }"></div>

<script type="text/html" id="myTemplate">
  <span data-bind="text: name"></span>
</script>

Custom Bindings

ko.bindingHandlers
Allows you to define your own custom bindings to encapsulate reusable UI logic.

Example:

ko.bindingHandlers.fadeVisible = {
  init: function(element, valueAccessor) {
    // Initially set the element to be instantly visible/hidden depending on the value
    var shouldDisplay = valueAccessor();
    $(element).toggle(shouldDisplay);
  },
  update: function(element, valueAccessor) {
    // Whenever the value subsequently changes, slowly fade the element in or out
    var shouldDisplay = valueAccessor();
    shouldDisplay ? $(element).fadeIn() : $(element).fadeOut();
  }
};

<div data-bind="fadeVisible: isVisible"></div>

Utilities and Extensions

Utilities

ko.utils.arrayForEach( array, callback )
Iterates over an array, executing a callback for each item.

Example:

ko.utils.arrayForEach(['A', 'B', 'C'], function(item) {
  console.log(item);
});

ko.utils.arrayMap( array, callback )
Transforms an array by applying a callback function to each item and returning a new array with the results.

Example:

var doubled = ko.utils.arrayMap([1, 2, 3], function(item) {
  return item * 2;
});
console.log(doubled); // Outputs: [2, 4, 6]

Extenders

extenders
Allow you to add custom functionality to observables.

Example:

ko.extenders.numeric = function(target, precision) {
  var result = ko.computed({
    read: target,
    write: function(newValue) {
      var current = target(),
          roundingMultiplier = Math.pow(10, precision),
          newValueAsNum = isNaN(newValue) ? 0 : parseFloat(+newValue),
          valueToWrite = Math.round(newValueAsNum * roundingMultiplier) / roundingMultiplier;

      if (valueToWrite !== current) {
        target(valueToWrite);
      } else {
        if (newValue !== current) {
          target.notifySubscribers(valueToWrite);
        }
      }
    }
  }).extend({ notify: 'always' });

  result(target());
  return result;
};

var price = ko.observable(123.456).extend({ numeric: 2 });