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JS Questions

Explain event delegation

When an event is fired from an element, the event will be bubbled up to its parent nodes. However, the original element where the event occurs, called 'target', stays the same in the event object. Using the target property, we can always keep tracking which element actually causes an event captured by its parent, and it can help use reduce the number of event handlers as we sometimes don't need to add event listeners for every element.

Explain how this works in JavaScript

Not answered yet

Explain how prototypal inheritance works

Not answered yet

What do you think of AMD vs CommonJS?

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Explain why the following doesn't work as an IIFE: function foo(){ }();.

Not answered yet

What needs to be changed to properly make it an IIFE?
(function foo() { })(); // or
(function foo() { }());

What's the difference between a variable that is: null, undefined or undeclared?

How would you go about checking for any of these states?

Not answered yet

What is a closure, and how/why would you use one?

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What's a typical use case for anonymous functions?

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How do you organize your code? (module pattern, classical inheritance?)

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What's the difference between host objects and native objects?

  • Host objects: what an environment(browser, Node.js, etc) provides
  • Native objects: what JavaScript provides

Difference between: function Person(){}, var person = Person(), and var person = new Person()?

Not answered yet

What's the difference between .call and .apply?

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Explain Function.prototype.bind.

Not answered yet

When would you use document.write()?

When someone gives me one million dollar for doing it.

What's the difference between feature detection, feature inference, and using the UA string?

  • Feature detection: directly check if a feature is implemented
if (Promise) {
  let a = Promise.resolve('hello');
}
  • Feature inference: infer if a feature is implemented by checking other properties
if (MozSmsMessage) {
  // guess it must be Firefox...
}
  • UA string: UA stands for UserAgent, which a browser natively exposes to scripts and HTTP header
console.log(navigator.userAgent); // "Mozilla/5.0 (Macintosh; ..."

Explain AJAX in as much detail as possible.

Not answered yet

Explain how JSONP works (and how it's not really AJAX).

A JSONP response contains a callback function usually written in JavaScript, and when the response is flushed, the callback will be launched. It's more like script tag injection, rather than AJAX.

Have you ever used JavaScript templating?

Yes.

If so, what libraries have you used?

Handlebars, Mustache, etc.

Explain "hoisting".

Not answered yet

Describe event bubbling.

It's when an event is bubbled into container elements, in the higher level of a DOM tree.

What's the difference between an "attribute" and a "property"?

  • Attribute: specified in HTML, always in the form of string
  • Property: specified in DOM object, can have any type of JavaScript

Why is extending built-in JavaScript objects not a good idea?

Not answered yet

Difference between document load event and document ready event?

  • document ready: when a HTML document is loaded and rendered
  • document load: when a HTML document and assets in the document are all loaded and rendered

What is the difference between == and ===?

Not answered yet

Explain the same-origin policy with regards to JavaScript.

Same-origin means having same host, port and protocol(HTTP or HTTPS). If a script in the different origin should be accessed, we can consider using CORS.

Make this work:

duplicate([1,2,3,4,5]); // [1,2,3,4,5,1,2,3,4,5]
let duplicate = (arr) => arr.concat(arr);

Why is it called a Ternary expression, what does the word "Ternary" indicate?

Not answered yet

What is "use strict";? what are the advantages and disadvantages to using it?

Advantages

  • Cannot assign a value to an undefined global variable
  • Fire TypeError for not-allowed assignments
  • this in a normal function refers to undefined, instead of global

In short, it secures JavaScript.

Disadvantage

  • When using global strict mode and concatenating the script with other scripts not using strict mode, the other scripts can be broken.

Create a for loop that iterates up to 100 while outputting "fizz" at multiples of 3, "buzz" at multiples of 5 and "fizzbuzz" at multiples of 3 and 5

Not answered yet

Why is it, in general, a good idea to leave the global scope of a website as-is and never touch it?

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Why would you use something like the load event? Does this event have disadvantages? Do you know any alternatives, and why would you use those?

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Explain what a single page app is and how to make one SEO-friendly.

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What is the extent of your experience with Promises and/or their polyfills?

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What are the pros and cons of using Promises instead of callbacks?

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What are some of the advantages/disadvantages of writing JavaScript code in a language that compiles to JavaScript?

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What tools and techniques do you use debugging JavaScript code?

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What language constructions do you use for iterating over object properties and array items?

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Explain the difference between mutable and immutable objects.

What is an example of an immutable object in JavaScript?
What are the pros and cons of immutability?
How can you achieve immutability in your own code?

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Explain the difference between synchronous and asynchronous functions.

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What is event loop?

What is the difference between call stack and task queue?

Not answered yet