Knockoutjs

Knockout.js is an open-source JavaScript library that implements the Model-View-ViewModel (MVVM) pattern to create rich, responsive user interfaces through declarative bindings. It enables automatic UI updates by tracking dependencies between observable data and the DOM.

Core concepts include: observables (ko.observable) for values that notify dependents on change; observable arrays (ko.observableArray) for dynamic lists; computed values (ko.computed or ko.pureComputed) for derived data; and a two-way data-binding system driven by HTML attributes such as data-bind. Bindings connect UI elements to view-model properties, and templates support dynamic lists and conditionals via foreach, if, and with bindings.

Usage typically involves including knockout.js, defining a JavaScript ViewModel with observable properties, and applying bindings with ko.applyBindings(viewModel). Bindings might synchronize an input field to an observable with the value binding and display text with the text binding.

Knockout integrates with jQuery and can operate alongside other libraries without requiring a full framework. It is well-suited for small to medium-sized applications and for progressively enhancing existing pages. It is not a full client-side framework and does not provide built-in routing or data access layers, so larger applications may require additional structure by the developer or combination with other tools.

History: Knockout.js was created by Steve Sanderson and released in 2010 as an open-source project. It gained early popularity for providing two-way data binding without imposing a large framework. Over time its prominence waned with the rise of modern frameworks, but it remains in use in legacy projects and for certain scenarios.