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.
How does “this” work in JS?
There’s no simple explanation for this; it is one of the most confusing concepts in JavaScript. A hand-wavey explanation is that the value of this depends on how the function is called. I have read many explanations on this online, and I found Arnav Aggrawal’s explanation to be the clearest. The following rules are applied:
- 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.
For an in-depth explanation, do check out his article on Medium.
Can you give an example of one of the ways that working with this has changed in ES6?
ES6 allows you to use arrow functions which uses the enclosing lexical scope. This is usually convenient, but does prevent the caller from controlling context via .call or .apply—the consequences being that a library such as jQuery will not properly bind this in your event handler functions. Thus, it’s important to keep this in mind when refactoring large legacy applications.
How does prototypal inheritance work?
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.
Example of Prototypal Inheritance:
We already have a build-in Object.create, but if you were to provide a polyfill for it, that might look like:
if (typeof Object.create !== 'function') {
Object.create = function (parent) {
function Tmp() {}
Tmp.prototype = parent;
return new Tmp();
};
}
const Parent = function() {
this.name = "Parent";
}
Parent.prototype.greet = function() { console.log(“hello from Parent”); }
const child = Object.create(Parent.prototype);
child.cry = function() {
console.log(“waaaaaahhhh!”);
}
child.cry(); // Outputs: waaaaaahhhh!
child.greet(); // Outputs: hello from Parent
Things to note are:
• .greet is not defined on the child, so the engine goes up the prototype chain and finds .greet off the inherited from Parent.
- We need to call Object.create in one of following ways for the prototype methods to be inherited:
- Object.create(Parent.prototype);
- Object.create(new Parent(null));
- Object.create(objLiteral);
- Currently, child.constructor is pointing to the Parent:
child.constructor
ƒ () {
this.name = "Parent";
}
child.constructor.name
"Parent"
• If we’d like to correct this, one option would be to do:
function Child() {
Parent.call(this);
this.name = 'child';
}
Child.prototype = Parent.prototype; Child.prototype.constructor = Child;
const c = new Child();
c.cry(); // Outputs: waaaaaahhhh!
c.greet(); // Outputs: hello from Parent
c.constructor.name; // Outputs: "Child"
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. An example:
// Don't add JS syntax to this code block to prevent Prettier from formatting it.
const foo = void function bar() { return 'foo'; }();
console.log(foo); // undefined
What’s the difference between a variable that is: null, undefined or undeclared? How would you go about checking for any of these states?
Undeclared variables are created when you assign a value to an identifier that is not previously created using var, let or const. Undeclared variables will be defined globally, outside of the current scope. In strict mode, a ReferenceError will be thrown when you try to assign to an undeclared variable. Undeclared variables are bad just like how global variables are bad. Avoid them at all cost! To check for them, wrap its usage in a try/catch block.
function foo() {
x = 1; // Throws a ReferenceError in strict mode
}
foo();
console.log(x); // 1
A variable that is undefined is a variable that has been declared, but not assigned a value. It is of type undefined. If a function does not return any value as the result of executing it is assigned to a variable, the variable also has the value of undefined. To check for it, compare using the strict equality (===) operator or typeof which will give the ‘undefined’ string. Note that you should not be using the abstract equality operator to check, as it will also return true if the value is null.
var foo;
console. log(foo); // undefined
console. log(foo === undefined); // true
console. log(typeof foo === ‘undefined’); // true
console.log(foo == null); // true. Wrong, don’t use this to check!
function bar() {}
var baz = bar();
console.log(baz); // undefined
A variable that is null will have been explicitly assigned to the null value. It represents no value and is different from undefined in the sense that it has been explicitly assigned. To check for null, simply compare using the strict equality operator. Note that like the above, you should not be using the abstract equality operator (==) to check, as it will also return true if the value is undefined.
var foo = null;
console. log(foo === null); // true
console. log(typeof foo === ‘object’); // true
console.log(foo == undefined); // true. Wrong, don’t use this to check!
As a personal habit, I never leave my variables undeclared or unassigned. I will explicitly assign null to them after declaring if I don’t intend to use it yet. If you use a linter in your workflow, it will usually also be able to check that you are not referencing undeclared variables.
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.
const a = [1, 2, 3];
const doubled = a.forEach((num, index) => {
// Do something with num and/or index.
});
// doubled = undefined
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.
const a = [1, 2, 3];
const doubled = a.map(num => {
return num * 2;
});
// doubled = [2, 4, 6]
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.
(function() {
// Some code here.
})();
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.
setTimeout(function() {
console.log(‘Hello world!’);
}, 1000);
Arguments to functional programming constructs or Lodash (similar to callbacks).
const arr = [1, 2, 3];
const double = arr.map(function(el) {
return el * 2;
});
console.log(double); // [2, 4, 6]
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).
function Person(name) {
this.name = name;
}
var person = Person(‘John’);
console. log(person); // undefined
console. log(person.name); // Uncaught TypeError: Cannot read property ‘name’ of undefined
var person = new Person(‘John’);
console. log(person); // Person { name: “John” }
console. log(person.name); // “john”
What’s the difference between .call and .apply?
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.
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.
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.
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.
