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Knockout JS Interview Questions and Answers

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Freshers / Beginner level questions & answers

Ques 1. What is Knockout JS?

Knockout JS is a JavaScript library that helps you to create rich, responsive display and editor user interfaces with a clean underlying data model.

Example:

var viewModel = { name: 'John', age: 25 }; ko.applyBindings(viewModel);

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Ques 2. What is an observable in Knockout JS?

An observable in Knockout JS is an object that can notify subscribers about changes, allowing the automatic updating of UI elements.

Example:

var observableValue = ko.observable('Initial value'); observableValue.subscribe(function(newValue) { console.log('New value:', newValue); });

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Ques 3. What is the purpose of the 'data-bind' attribute in Knockout JS?

The 'data-bind' attribute is used to associate HTML elements with Knockout JS data-bindings, enabling the establishment of a connection between the UI and the underlying view model.

Example:

; var viewModel = { message: 'Hello, Knockout!' }; ko.applyBindings(viewModel);

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Ques 4. How can you handle click events in Knockout JS?

You can use the 'click' binding to associate a function with a click event on an HTML element.

Example:

Click me; var viewModel = { handleClick: function() { alert('Button clicked!'); } };

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Ques 5. What is the purpose of the 'text' binding in Knockout JS?

The 'text' binding is used to set the text content of an HTML element based on the value of the associated observable or expression.

Example:

; var viewModel = { message: 'Hello, Knockout!' };

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Ques 6. What is the purpose of the 'value' binding in Knockout JS?

The 'value' binding is used to bind an input, select, or textarea element's value to an observable, allowing two-way data binding.

Example:

; var viewModel = { userInput: ko.observable('Initial value') };

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Ques 7. Explain the 'subscribe' method in Knockout JS.

The 'subscribe' method is used to register a callback function that will be called whenever the associated observable's value changes.

Example:

var myObservable = ko.observable('Initial value'); myObservable.subscribe(function(newValue) { console.log('New value:', newValue); });

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Ques 8. How does the 'checked' binding work in Knockout JS?

The 'checked' binding is used to bind the checked state of a checkbox or radio input to an observable, enabling two-way data binding.

Example:

; var viewModel = { isChecked: ko.observable(true) };

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Ques 9. What is the purpose of the 'if' binding in Knockout JS?

The 'if' binding is used to conditionally render or remove an HTML element based on the truthiness of the associated observable or expression.

Example:

Content to show
; var viewModel = { shouldShowContent: ko.observable(true) };

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Ques 10. What is the purpose of the 'options' binding in Knockout JS?

The 'options' binding is used to generate a set of 'option' elements based on an array or object and bind the selected value to an observable.

Example:

; var viewModel = { countries: ['USA', 'Canada', 'UK'], selectedCountry: ko.observable('USA') };

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Intermediate / 1 to 5 years experienced level questions & answers

Ques 11. Explain two-way data binding in Knockout JS.

Two-way data binding in Knockout JS ensures that when the UI changes, the underlying data model is automatically updated, and vice versa.

Example:

HTML: ; JavaScript: var viewModel = { name: ko.observable('John') };

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Ques 12. How does the 'foreach' binding work in Knockout JS?

The 'foreach' binding is used to iterate over an array and generate content for each item in the array within the specified HTML element.

Example:

HTML: ; JavaScript: var items = ko.observableArray(['Item 1', 'Item 2']); ko.applyBindings({ items: items });

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Ques 13. Explain the 'visible' binding in Knockout JS.

The 'visible' binding is used to control the visibility of an HTML element based on the truthiness of the associated observable or expression.

Example:

Visible Content
; var viewModel = { showElement: ko.observable(true) };

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Ques 14. Explain the concept of 'template' binding in Knockout JS.

The 'template' binding is used to render content based on a template defined elsewhere in the HTML or within the view model.

Example:

; ; var viewModel = { person: { name: 'John' } };

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Ques 15. What is the purpose of the 'attr' binding in Knockout JS?

The 'attr' binding is used to set or remove one or more attributes of an HTML element based on the value of the associated observable or expression.

Example:

; var viewModel = { imageUrl: 'path/to/image.jpg', imageAlt: 'Image Alt Text' };

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Ques 16. How can you handle key events in Knockout JS?

You can use the 'event' binding to handle various events, including key events, on HTML elements.

Example:

; var viewModel = { handleKeyPress: function(data, event) { console.log('Key pressed:', event.key); } };

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Ques 17. Explain the 'css' binding in Knockout JS.

The 'css' binding is used to apply or remove CSS classes to an HTML element based on the truthiness of the associated observable or expression.

Example:

Content
; var viewModel = { isActive: ko.observable(true), isDisabled: ko.observable(false) };

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Ques 18. What is the purpose of the 'foreach' and 'as' combination in Knockout JS?

The 'foreach' and 'as' combination is used to alias the variable name used to represent each item in an array when using the 'foreach' binding.

Example:

; var items = ko.observableArray(['Item 1', 'Item 2']);

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Ques 19. Explain the purpose of the 'hasfocus' binding in Knockout JS.

The 'hasfocus' binding is used to bind an observable to the focus state of an element, allowing you to track and control focus programmatically.

Example:

; var viewModel = { isFocused: ko.observable(true) };

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Ques 20. How can you handle submit events in Knockout JS?

You can use the 'submit' binding to associate a function with the submit event of a form element.

Example:

  ; var viewModel = { handleSubmit: function() { alert('Form submitted!'); } };

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Ques 21. Explain the 'with' binding in Knockout JS.

The 'with' binding is used to change the context for descendant elements, allowing you to bind against a different object.

Example:

; var viewModel = { person: { name: 'John' } };

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Experienced / Expert level questions & answers

Ques 22. Explain the concept of computed observables.

Computed observables in Knockout JS are functions that automatically update whenever the underlying observables they depend on change.

Example:

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

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Ques 23. Explain the concept of custom bindings in Knockout JS.

Custom bindings in Knockout JS allow you to create reusable, encapsulated behaviors for handling specific aspects of the UI.

Example:

ko.bindingHandlers.fadeVisible = { init: function(element, valueAccessor) { var value = valueAccessor(); $(element).toggle(ko.unwrap(value)); }, update: function(element, valueAccessor) { var value = valueAccessor(); ko.unwrap(value) ? $(element).fadeIn() : $(element).fadeOut(); } };

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Ques 24. What is the purpose of the 'afterRender' callback in Knockout JS?

The 'afterRender' callback is used with the 'foreach' binding to execute a function after each item in the array is rendered in the UI.

Example:

; var items = ko.observableArray(['Item 1', 'Item 2']); function myCallback(elements) { console.log('Rendered elements:', elements); }

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Ques 25. Explain the 'templateOptions' property in Knockout JS.

The 'templateOptions' property allows you to pass additional options to a template specified in the 'template' binding.

Example:

;  Details; var viewModel = { person: { name: 'John' } };

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