JavaScript Interview Questions

Theory Section

How does the "this" keyword work?

First step is to find the place where the function was called.
Second step is to check questions from top to the bottom and stop if the rule is applied.

Is a function called with new?

Answer: this is a newly constructed object

function Person(name) {
  this.name = name;
}

const person = new Person("John");
console.log(person.name); // John

Is call, apply or bind was used to call the function?

Answer: this is a specified thisArg parameter

function getName() {
  return this.name;
}

const person = {
  name: "John"
};

console.log(getName.call(person)); // John
console.log(getName.apply(person)); // John

const boundGetName = getName.bind(person);

console.log(boundGetName()); // John

Is a function called with context object.

Answer: this is a context object

function getName() {
  return this.name;
}

const person = {
  name: "John",
  getName
};

// "this" = person (context object) for the `getName` function
console.log(person.getName()); // John

Default case:

Answer in strict mode: this is an undefined

var name = "globalName";

function getName() {
  "use strict";
  return this.name;
}

console.log(getName()); // TypeError: Cannot read property 'name' of undefined

Answer in non-strict mode: this is a global object

var name = "globalName";

function getName() {
  return this.name;
}

console.log(getName()); // globalName

Note:

Arrow function doesn't have own this. It takes it from the outside. See arrow functions

What is an arrow function?

Syntax:

// arrow function
const sum = (a, b) => a + b;

// regular function
function sum(a, b) {
  return a + b;
}

No this

The this value of the enclosing lexical scope is used.

It's better not to use an arrow function as a method.

const company = {
  name: "Awesome",
  employees: ["John", "Mike", "Joe"],

  printEmployees() {
    this.employees.forEach(employee => {
      // "this" = company, is taken from the outside
      console.log(`${this.name}: ${employee}`);
    });
  }
};

company.printEmployees();

// Output:
// Awesome: John
// Awesome: Mike
// Awesome: Joe

No arguments

Takes it from the enclosing scope.

Not a big deal as rest parameters could be used alternatively.

function sum() {
  // "arguments" of the enclosing scope
  const helper = () => arguments[0] + arguments[1];
  return helper();
}

console.log(sum(2, 3)); // 5

No super

Takes it from the enclosing scope.

class Vehicle {
  start() {
    console.log("Vehicle: start");
  }
}

class Car extends Vehicle {
  start() {
    console.log("Insert and turn the key");

    const helper = () => {
      // takes "super" from the outer function
      super.start();
      console.log("Car: started");
    };

    helper();
  }
}

const car = new Car();
car.start();

// Output:
// Insert and turn the key
// Vehicle: start
// Car: started

Can't be used with new

const Foo = () => {};
const foo = new Foo(); // TypeError: Foo is not a constructor

No prototype property

const Person = () => {};
console.log(Person.prototype); // undefined

What is a "hoisting"?

Variables (defined with "var") and function declarations are processed before any part your code is executed.

That statement has actually two parts:

var name = "John";

var name - declaration, processed during the compilation phase
a = "John" - assignment, processed during execution phase

Example:

// hoisting: function could be called before declaration
getName();

function getName() {
  // hoisting: could be used before variable declaration
  console.log(name); // undefined

  var name = "John";
}

Note:

hoisting works only per-scope
function expressions are not hoisted
functions are hoisted before variables

What built-in types does JavaScript have?

null
undefined
boolean
number
string
object
symbol (added in ES6)

All types are primitives except an object

What are the rest parameters and spread operators?

Rest Parameter

Rest parameter allows us to collect indefinite number of arguments.

function sum(...args) {
  // args is an array of parameters
  return args.reduce((previous, current) => previous + current);
}

console.log(sum(1, 2, 3, 4, 5)); // 15

rest parameters vs "arguments"

"arguments" is not a real array. sort, filter, reduce, map and other array functions couldn't be used with it. Rest parameter is an Array instance.
Arrow functions don't have "arguments". Rest parameters should be used for them.
"arguments" object contains all arguments passed to the function, while the rest parameter contains only the remaining part without parameters with separate names

Note:

The rest parameter have to be the last argument in the function.

Spread Operator

Allows an iterable (array, string, etc) to be expanded in the next places.

// function call
fn(...iterableObj);

// array
[...iterableObj, 1, 2];

// object literal
const clonedObj = { ...obj };

fn.apply(null, args) is the same as fn(...args)

What are the "call" and "apply" functions? "call" vs "apply"?

These methods are used to call a function with a given this and arguments.

"call" vs "apply":

call() accepts an argument list, while apply() accepts an array of arguments

function getInfo(age, position) {
  return `${this.name}, ${age}, ${position}`;
}

const person = { name: "John" };

getInfo.call(person, 34, "JS Developer"); // John, 34, JS Developer
getInfo.apply(person, [34, "JS Developer"]); // John, 34, JS Developer

What is an "instanceof" operator? How does it work?

obj instanceof constructor checks whether constructor.prototype equals to one of the prototypes in the obj prototype chain.

class Vehicle {}
class Car extends Vehicle {}

const car = new Car();

car instanceof Car; // true
car instanceof Vehicle; // true
car instanceof Object; // true

// under the hood
car.__proto__ === Car.prototype; // true
car.__proto__.__proto__ === Vehicle.prototype; // true
car.__proto__.__proto__.__proto__ === Object.prototype; // true

Note:

instanceof behavior could be customized by adding static Symbol.hasInstance method.

class Person {
  static [Symbol.hasInstance](value) {
    if ("name" in value && "age" in value) {
      return true;
    }

    return false;
  }
}

console.log({ name: "John", age: "29" } instanceof Person); // true

What is a closure?

A closure is a function that remembers its outer variables and can access them.

function createCounter() {
  let count = 0;
  return function() {
    // takes `count` from the outer lexical environment
    return count++;
  };
}

let firstCounter = createCounter();
let secondCounter = createCounter();

console.log(firstCounter()); // 0
console.log(firstCounter()); // 1
console.log(firstCounter()); // 2

console.log(secondCounter()); // 0

Lexical Environment

Lexical Environment consists of:

Environment Record (local variables and this for the function environment)
reference to an outer Lexical Environment

Lexical Environment is created for each function call and evaluated code block ({ ... }).

To access a variable the inner Lexical Environment is searched first, then the outer one and so on until a variable is found or global environment is reached.

All functions have internal property [[Environment]] with a reference to the Lexical Environment that the function was closed over (creation place).

What is an `IIFE`?

Immediately Invoked Function Expression is a function that runs as soon as it is defined.

The variables within IIFE can not be accessed from outside and not polluting the global scope.

(function() {
  const count = 5;

  console.log(count); // 5
})();

console.log(count); // ReferenceError: count is not defined

What is a `bind` function?

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.

const person = {
  name: "John",
  getName: function() {
    return this.name;
  }
};

const getName = person.getName;
// this = undefined in strict mode or
// this = global object in non-strict mode
console.log(getName()); // undefined

const boundGetName = getName.bind(person);
// this = person
console.log(boundGetName()); // John

Note:

Partial Application could be implemented using bind function

What is a "partial application"?

The process of fixing a number of arguments to a function, producing another function of smaller arity (number of arguments).

function multiply(x, y) {
  return x * y;
}

const double = multiply.bind(null, 2);
const triple = multiply.bind(null, 3);

console.log(double(4)); // 8
console.log(triple(3)); // 9

null was passed to the bind function as this argument because we don't use this in our multiply function. this will be equal to:

null in strict mode
global object in non-strict mode

How to apply some arguments without changing this?

function partial(fn, ...partialArgs) {
  return function(...args) {
    return fn.call(this, ...partialArgs, ...args);
  };
}

const person = {
  name: "John",
  getInfo(company, year) {
    return `${company}: ${this.name}, ${year}`;
  }
};

person.newGetInfo = partial(person.getInfo, "Awesome");
console.log(person.newGetInfo(2019)); // Awesome: John, 2019

Also, _.partial and _.partialRight functions could be used from the lodash library.

What is a "currying"?

Currying is the technique of translating the evaluation of a function that takes multiple arguments into evaluating a sequence of functions, each with a single argument.

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) {
      return fn.apply(this, args);
    } else {
      return function(...otherArgs) {
        return curried.call(this, ...args, ...otherArgs);
      };
    }
  };
}

function sum(a, b, c) {
  return a + b + c;
}

const curriedSum = curry(sum);

// currying by definition
console.log(curriedSum(1)(2)(3)); // 6

// additional opportunity to use
console.log(curriedSum(1, 2, 3)); // 6
console.log(curriedSum(1, 2)(3)); // 6
console.log(curriedSum(1)(2, 3)); // 6

According to the definition, currying should convert sum(a, b, c) to the sum(a)(b)(c).

But most of the time other libraries implement it in a way, that gives an ability to pass multiple arguments at a time as in the example above.

Partial Application vs Currying:

partial application can take as many arguments as desired but curried function takes one argument at a time

Note:

_.curry or _.curryRight from the lodash library could be used to apply currying technique.

"var" vs "let" vs "const"?

const is the same as let without ability to change its value.

let and const have block scope but var doesn't. Variables, declared with var are function-wide or global.

if (true) {
  var first = true;
  let second = true;
}

console.log(first); // true, the variable defined with "var" is accessible outside "if" scope
console.log(second); // ReferenceError: second is not defined

{
  var first = true;
  let second = true;
}

console.log(first); // true, the variable defined with "var" is accessible outside `{ ... }` scope
console.log(second); // ReferenceError: second is not defined

// defined with "var"
for (var firstIdx = 0; firstIdx < 5; firstIdx++) {}
// defined with "let"
for (let secondIdx = 0; secondIdx < 5; secondIdx++) {}

console.log(firstIdx); // 5 - visible outside for loop
console.log(secondIdx); // ReferenceError: secondIdx is not defined

function fn() {
  // variables defined inside the function are not accessible outside it
  // no matter what keyword was used to define it ("var", "let" or "const")
  var message = "some message";
}

console.log(message); // ReferenceError: message is not defined

variables defined with "var" are hoisted (see hoisting), while defined with "let" and "const" are not

What is a "prototype"?

Objects have a hidden property [[Prototype]], that references another object or null.

When trying to access a property of an object, the property will not only be sought on the object but on the prototype of the object, the prototype of the prototype, and so on until either a property with a matching name is found or the end of the prototype chain is reached.

`[[Prototype]]` vs `Func.prototype`

Func.prototype specifies the [[Prototype]] to be assigned to the objects created by the Func function when it is used a constructor (called with a new operator).

The value of the func.prototype should be an object or null.

function Person(name) {
  this.name = name;
}

Person.prototype.getName = function() {
  return this.name;
};

// Person.prototype object will be assigned as a [[Prototype]] for a person object
const person = new Person("John");

// takes getName function from the [[Prototype]] object
console.log(person.getName()); // John

How to set / get a "prototype" of the object?

__proto__ is a historical setter / getter for [[Prototype]], which is deprecated now.

const shape = {
  color: "red"
};

const circle = {
  radius: 5
};

// set the prototype object for the circle
circle.__proto__ = shape;

// takes from the object
console.log(circle.radius); // 5

// takes from the prototype object (shape)
console.log(circle.color); // red

// shape is the prototype of the circle
console.log(circle.__proto__ === shape); // true

Object.getPrototypeOf() / Object.setPrototypeOf()

const shape = {
  color: "red"
};

const circle = {
  radius: 5
};

// set the prototype object for the circle
Object.setPrototypeOf(circle, shape);

// takes from the object
console.log(circle.radius); // 5

// takes from the prototype object (shape)
console.log(circle.color); // red

// shape is the prototype of the circle
console.log(Object.getPrototypeOf(circle) === shape); // true

Object.create() creates a new object with specified [[Prototype]]

const person = {
  getName: function() {
    return this.name;
  }
};

// person object will be assigned as a [[Prototype]] for a me object
const me = Object.create(person);
me.name = "John";

// takes getName function from the [[Prototype]] object
console.log(me.getName()); // John

Object.prototype

new Object() or { ... } has an Object.prototype as a [[Prototype]].

All built-in prototypes have Object.prototype on the top.

const obj = {};

console.log(obj.__proto__ === Object.prototype); // true
console.log(obj.__proto__.__proto__); // null

const arr = [1, 2, 3];

console.log(arr.__proto__ === Array.prototype); // true
console.log(arr.__proto__.__proto__ === Object.prototype); // true
console.log(arr.__proto__.__proto__.__proto__); // null

Practice Section

What's the output?

typeof null;

A: "null"
B: "object"
C: "undefined"

Answer

Answer: B (`"object"`)

By specification:

typeof null === "object"; // true

What's the output?

function Person() {}

const person = new Person();
Person.prototype = {};

console.log(person instanceof Person); // ???

A: true
B: false

Answer

Answer: B (`false`)

obj instanceof constructor checks whether constructor.prototype equals to one of the prototypes in the obj prototype chain.

The Person.prototype was changed after person creation, but it doesn't change the [[Prototype]] of the created object.

person.__proto__ === Person.prototype; // false

What's the output?

const person = {
  name: "John",
  getName: () => {
    return this.name;
  }
};

console.log(person.getName()); // ???

A: John
B: TypeError
C: undefined

Answer

Answer: C (`undefined`)

Arrow functions have no this, so the value of the enclosing lexical scope is used.

It's a global object (window) in that case. But the window doesn't have name property and returns undefined.

What's the output?

const value = new Boolean(false);
console.log(!!value); // ???

A: false
B: true
C: error

Answer

Answer: B (`true`)

new Boolean(false) expression creates an object wrapper for the boolean value and the value of !!object is always true.

typeof new Boolean(false); // "object"

!!{}; // true

What's the output?

const obj = {};

const key = "__proto__";
obj[key] = 13;

console.log(obj[key]); // ???

A: error
B: [object Object]
C: 13

Answer

Answer: B (`[object Object]`)

__proto__ value should be an object or null. Other values will be ignored. By default the [[Prototype]] for an object is an Object.prototype.

const obj = {};

const key = "__proto__";
obj[key] = 13;

console.log(obj[key] === Object.prototype); // true

To solve that problem Object.create(null) could be used. That expression creates an object without a [[Prototype]].

// obj has no [[Prototype]]
const obj = Object.create(null);

const key = "__proto__";
obj[key] = 13;

console.log(obj[key]); // 13

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JavaScript Interview Questions

Theory Section

How does the "this" keyword work?

What is an arrow function?

What is a "hoisting"?

What built-in types does JavaScript have?

What are the rest parameters and spread operators?

Rest Parameter

Spread Operator

What are the "call" and "apply" functions? "call" vs "apply"?

What is an "instanceof" operator? How does it work?

What is a closure?

What is an IIFE?

What is a bind function?

What is a "partial application"?

What is a "currying"?

"var" vs "let" vs "const"?

What is a "prototype"?

[[Prototype]] vs Func.prototype

How to set / get a "prototype" of the object?

Object.prototype

Practice Section

What's the output?

Answer: B ("object")

What's the output?

Answer: B (false)

What's the output?

Answer: C (undefined)

What's the output?

Answer: B (true)

What's the output?

Answer: B ([object Object])

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What is an `IIFE`?

What is a `bind` function?

`[[Prototype]]` vs `Func.prototype`

Answer: B (`"object"`)

Answer: B (`false`)

Answer: C (`undefined`)

Answer: B (`true`)

Answer: B (`[object Object]`)

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