Javascript Flashcards
Explain event delegation
“Event delegation is a technique involving adding event listeners to a parent element instead of adding them to the descendant elements. The listener will fire whenever the event is triggered on the descendant elements due to event bubbling up the DOM. The benefits of this technique are:
Memory footprint goes down because only one single handler is needed on the parent element, rather than having to attach event handlers on each descendant.
There is no need to unbind the handler from elements that are removed and to bind the event for new elements.
“
Explain how “this” works in JavaScript
- If the new keyword is used when calling the function, this inside the function is a brand new object.
- If apply, call, or bind are used to call/create a function, this inside the function is the object that is passed in as the argument.
- If a function is called as a method, such as obj.method() — this is the object that the function is a property of.
- If a function is invoked as a free function invocation, meaning it was invoked without any of the conditions present above, this is the global object.
- In a browser, it is the window object.
- If in strict mode (‘use strict’), this will be undefined instead of the global object.
- If multiple of the above rules apply, the rule that is higher wins and will set the this value.
- If the function is an ES2015 arrow function, it ignores all the rules above and receives the this value of its surrounding scope at the time it is created.”
Explain how prototypal inheritance works
This is an extremely common JavaScript interview question. All JavaScript objects have a prototype property, that is a reference to another object. When a property is accessed on an object and if the property is not found on that object, the JavaScript engine looks at the object’s prototype, and the prototype’s prototype and so on, until it finds the property defined on one of the prototypes or until it reaches the end of the prototype chain. This behavior simulates classical inheritance, but it is really more of delegation than inheritance.
What do you think of AMD vs CommonJS?
“Both are ways to implement a module system, which was not natively present in JavaScript until ES2015 came along. CommonJS is synchronous while AMD (Asynchronous Module Definition) is obviously asynchronous. CommonJS is designed with server-side development in mind while AMD, with its support for asynchronous loading of modules, is more intended for browsers.
I find AMD syntax to be quite verbose and CommonJS is closer to the style you would write import statements in other languages. Most of the time, I find AMD unnecessary, because if you served all your JavaScript into one concatenated bundle file, you wouldn’t benefit from the async loading properties. Also, CommonJS syntax is closer to Node style of writing modules and there is less context-switching overhead when switching between client side and server side JavaScript development.
I’m glad that with ES2015 modules, that has support for both synchronous and asynchronous loading, we can finally just stick to one approach. Although it hasn’t been fully rolled out in browsers and in Node, we can always use transpilers to convert our code.”
Explain why the following doesn’t work as an IIFE: function foo(){ }();. What needs to be changed to properly make it an IIFE?
“IIFE stands for Immediately Invoked Function Expressions. The JavaScript parser reads function foo(){ }(); as function foo(){ } and ();, where the former is a function declaration and the latter (a pair of parentheses) is an attempt at calling a function but there is no name specified, hence it throws Uncaught SyntaxError: Unexpected token ).
Here are two ways to fix it that involves adding more parentheses: (function foo(){ })() and (function foo(){ }()). Statements that begin with function are considered to be function declarations; by wrapping this function within (), it becomes a function expression which can then be executed with the subsequent (). These functions are not exposed in the global scope and you can even omit its name if you do not need to reference itself within the body.
You might also use void operator: void function foo(){ }();. Unfortunately, there is one issue with such approach. The evaluation of given expression is always undefined, so if your IIFE function returns anything, you can’t use it.”
What’s the difference between a variable that is: null, undefined or undeclared? How would you go about checking for any of these states?
“A variable that is:
undeclared - assigned a value to an identifier that isn’t previously created using var, let, or const. in strict mode a ReferenceError will be thrown when you try to assign to an undeclared variable. To check for them, wrap its usage in a try/catch block
undefined - two types of undefined variables are if it has been declared but not assigned a value or if a function does not return any value as the result of executing, it is assigned to a variable - the variable also has a value of undefined. To check for it, compare using the strict equality (===) operator or typeof, which will give you the ‘undefined’ string.
null - explicitly assigned to the null value. represents no value and is different from undefined because it was assigned null intentionally. to check for null, use the strict equality operator (===)”
What is a closure, and how/why would you use one?
“A closure is the combination of a function and the lexical environment within which that function was declared. The word ““lexical”” refers to the fact that lexical scoping uses the location where a variable is declared within the source code to determine where that variable is available. Closures are functions that have access to the outer (enclosing) function’s variables—scope chain even after the outer function has returned.
Why would you use one?
Data privacy / emulating private methods with closures. Commonly used in the module pattern. Partial applications or currying."
Can you describe the main difference between a .forEach loop and a .map() loop and why you would pick one versus the other?
“To understand the differences between the two, let’s look at what each function does.
forEach
Iterates through the elements in an array.
Executes a callback for each element.
Does not return a value.
map
Iterates through the elements in an array.
““Maps”” each element to a new element by calling the function on each element, creating a new array as a result.
The main difference between .forEach and .map() is that .map() returns a new array. If you need the result, but do not wish to mutate the original array, .map() is the clear choice. If you simply need to iterate over an array, forEach is a fine choice.
“
What’s a typical use case for anonymous functions?
“They can be used in IIFEs to encapsulate some code within a local scope so that variables declared in it do not leak to the global scope.
As a callback that is used once and does not need to be used anywhere else. The code will seem more self-contained and readable when handlers are defined right inside the code calling them, rather than having to search elsewhere to find the function body.
Arguments to functional programming constructs or Lodash (similar to callbacks).
“
How do you organize your code? (module pattern, classical inheritance?)
“In the past, I’ve used Backbone for my models which encourages a more OOP approach, creating Backbone models and attaching methods to them.
The module pattern is still great, but these days, I use React/Redux which utilize a single-directional data flow based on Flux architecture. I would represent my app’s models using plain objects and write utility pure functions to manipulate these objects. State is manipulated using actions and reducers like in any other Redux application.
I avoid using classical inheritance where possible. When and if I do, I stick to these rules.”
What’s the difference between host objects and native objects?
“Native objects are objects that are part of the JavaScript language defined by the ECMAScript specification, such as String, Math, RegExp, Object, Function, etc.
Host objects are provided by the runtime environment (browser or Node), such as window, XMLHTTPRequest, etc.”
Difference between: function Person(){}, var person = Person(), and var person = new Person()?
“This question is pretty vague. My best guess at its intention is that it is asking about constructors in JavaScript. Technically speaking, function Person(){} is just a normal function declaration. The convention is to use PascalCase for functions that are intended to be used as constructors.
var person = Person() invokes the Person as a function, and not as a constructor. Invoking as such is a common mistake if the function is intended to be used as a constructor. Typically, the constructor does not return anything, hence invoking the constructor like a normal function will return undefined and that gets assigned to the variable intended as the instance.
var person = new Person() creates an instance of the Person object using the new operator, which inherits from Person.prototype. An alternative would be to use Object.create, such as: Object.create(Person.prototype).”
What’s the difference between .call and .apply?
“both methods
.call comma separated arguments
.apply array of arguments
Both .call and .apply are used to invoke functions and the first parameter will be used as the value of this within the function. However, .call takes in comma-separated arguments as the next arguments while .apply takes in an array of arguments as the next argument. An easy way to remember this is C for call and comma-separated and A for apply and an array of arguments.
function add(a, b) {
return a + b;
}
console. log(add.call(null, 1, 2)); // 3
console. log(add.apply(null, [1, 2])); // 3”
Explain Function.prototype.bind.
“Taken word-for-word from MDN:
The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
In my experience, it is most useful for binding the value of this in methods of classes that you want to pass into other functions. This is frequently done in React components.”
When would you use document.write()?
“document.write() writes a string of text to a document stream opened by document.open(). When document.write() is executed after the page has loaded, it will call document.open which clears the whole document ( and removed!) and replaces the contents with the given parameter value. Hence it is usually considered dangerous and prone to misuse.
There are some answers online that explain document.write() is being used in analytics code or when you want to include styles that should only work if JavaScript is enabled. It is even being used in HTML5 boilerplate to load scripts in parallel and preserve execution order! However, I suspect those reasons might be outdated and in the modern day, they can be achieved without using document.write(). Please do correct me if I’m wrong about this.
References
https://www.quirksmode.org/blog/archives/2005/06/three_javascrip_1.html
https://github.com/h5bp/html5-boilerplate/wiki/Script-Loading-Techniques#documentwrite-script-tag
“
What’s the difference between feature detection, feature inference, and using the UA string?
“Feature Detection
Feature detection involves working out whether a browser supports a certain block of code, and running different code depending on whether it does (or doesn’t), so that the browser can always provide a working experience rather crashing/erroring in some browsers. For example:
if ('geolocation' in navigator) {
// Can use navigator.geolocation
} else {
// Handle lack of feature
}
Modernizr is a great library to handle feature detection.
Feature Inference
Feature inference checks for a feature just like feature detection, but uses another function because it assumes it will also exist, e.g.:
if (document.getElementsByTagName) {
element = document.getElementById(id);
}
This is not really recommended. Feature detection is more foolproof.
UA String
This is a browser-reported string that allows the network protocol peers to identify the application type, operating system, software vendor or software version of the requesting software user agent. It can be accessed via navigator.userAgent. However, the string is tricky to parse and can be spoofed. For example, Chrome reports both as Chrome and Safari. So to detect Safari you have to check for the Safari string and the absence of the Chrome string. Avoid this method.
References
https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection
https://stackoverflow.com/questions/20104930/whats-the-difference-between-feature-detection-feature-inference-and-using-th
https://developer.mozilla.org/en-US/docs/Web/HTTP/Browser_detection_using_the_user_agent
“
Explain Ajax in as much detail as possible.
“Ajax (asynchronous JavaScript and XML) is a set of web development techniques using many web technologies on the client side to create asynchronous web applications. With Ajax, web applications can send data to and retrieve from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. By decoupling the data interchange layer from the presentation layer, Ajax allows for web pages, and by extension web applications, to change content dynamically without the need to reload the entire page. In practice, modern implementations commonly substitute use JSON instead of XML, due to the advantages of JSON being native to JavaScript.
The XMLHttpRequest API is frequently used for the asynchronous communication or these days, the fetch API.
References
https://en.wikipedia.org/wiki/Ajax_(programming)
https://developer.mozilla.org/en-US/docs/AJAX
“
What are the advantages and disadvantages of using Ajax?
“Advantages
Better interactivity. New content from the server can be changed dynamically without the need to reload the entire page.
Reduce connections to the server since scripts and stylesheets only have to be requested once.
State can be maintained on a page. JavaScript variables and DOM state will persist because the main container page was not reloaded.
Basically most of the advantages of an SPA.
Disadvantages
Dynamic webpages are harder to bookmark.
Does not work if JavaScript has been disabled in the browser.
Some webcrawlers do not execute JavaScript and would not see content that has been loaded by JavaScript.
Basically most of the disadvantages of an SPA.
“
Explain how JSONP works (and how it’s not really Ajax).
“JSONP (JSON with Padding) is a method commonly used to bypass the cross-domain policies in web browsers because Ajax requests from the current page to a cross-origin domain is not allowed.
JSONP works by making a request to a cross-origin domain via a tag and usually with a callback query parameter, for example: https://example.com?callback=printData. The server will then wrap the data within a function called printData and return it to the client.
<!-- https://mydomain.com -->
<script>
function printData(data) {
console.log(`My name is ${data.name}!`);
}
// File loaded from https://example.com?callback=printData
printData({ name: 'Yang Shun' });
The client has to have the printData function in its global scope and the function will be executed by the client when the response from the cross-origin domain is received.
JSONP can be unsafe and has some security implications. As JSONP is really JavaScript, it can do everything else JavaScript can do, so you need to trust the provider of the JSONP data.
These days, CORS is the recommended approach and JSONP is seen as a hack.
References
https://stackoverflow.com/a/2067584/1751946”
Have you ever used JavaScript templating? If so, what libraries have you used?
“Yes. Handlebars, Underscore, Lodash, AngularJS, and JSX. I disliked templating in AngularJS because it made heavy use of strings in the directives and typos would go uncaught. JSX is my new favorite as it is closer to JavaScript and there is barely any syntax to learn. Nowadays, you can even use ES2015 template string literals as a quick way for creating templates without relying on third-party code.
const template = `<div>My name is: ${name}</div>`;
However, do be aware of a potential XSS in the above approach as the contents are not escaped for you, unlike in templating libraries."
Explain “hoisting”.
“Hoisting is a term used to explain the behavior of variable declarations in your code. Variables declared or initialized with the var keyword will have their declaration ““moved”” up to the top of the current scope, which we refer to as hoisting. However, only the declaration is hoisted, the assignment (if there is one), will stay where it is.
Note that the declaration is not actually moved - the JavaScript engine parses the declarations during compilation and becomes aware of declarations and their scopes. It is just easier to understand this behavior by visualizing the declarations as being hoisted to the top of their scope. Let’s explain with a few examples.
// var declarations are hoisted. console.log(foo); // undefined var foo = 1; console.log(foo); // 1
// let/const declarations are NOT hoisted. console.log(bar); // ReferenceError: bar is not defined let bar = 2; console.log(bar); // 2 Function declarations have the body hoisted while the function expressions (written in the form of variable declarations) only has the variable declaration hoisted.
// Function Declaration
console.log(foo); // [Function: foo]
foo(); // 'FOOOOO'
function foo() {
console.log('FOOOOO');
}
console.log(foo); // [Function: foo]
// Function Expression
console.log(bar); // undefined
bar(); // Uncaught TypeError: bar is not a function
var bar = function() {
console.log('BARRRR');
};
console.log(bar); // [Function: bar]"
Describe event bubbling.
When an event triggers on a DOM element, it will attempt to handle the event if there is a listener attached, then the event is bubbled up to its parent and the same thing happens. This bubbling occurs up the element’s ancestors all the way to the document. Event bubbling is the mechanism behind event delegation.
What’s the difference between an “attribute” and a “property”?
Attributes are defined on the HTML markup but properties are defined on the DOM. To illustrate the difference, imagine we have this text field in our HTML: .
Why is extending built-in JavaScript objects not a good idea?
“Extending a built-in/native JavaScript object means adding properties/functions to its prototype. While this may seem like a good idea at first, it is dangerous in practice. Imagine your code uses a few libraries that both extend the Array.prototype by adding the same contains method, the implementations will overwrite each other and your code will break if the behavior of these two methods is not the same.
The only time you may want to extend a native object is when you want to create a polyfill, essentially providing your own implementation for a method that is part of the JavaScript specification but might not exist in the user’s browser due to it being an older browser.”
