20 Tricky ES6 Interview Questions to Revamp Your JavaScript Skills

ES6 refers to version 6 of the ECMA Script programming language. ECMA Script is the standardized name for JavaScript, and version 6 is the next version after version 5, which was released in 2011. It is a major enhancement to the JavaScript language and adds many more features intended to make large-scale software development easier. Both ES6 and ES2015 names used for that version of JavaScript that introduces arrow functions, classes, template strings, Promises and more.

Originally published on FullStack.Cafe - Never Fail Your Tech Interview Again

Q1: Could you explain the difference between ES5 and ES6

Topic: JavaScript
Difficulty: ⭐⭐⭐

• ECMAScript 5 (ES5): The 5th edition of ECMAScript, standardized in 2009. This standard has been implemented fairly completely in all modern browsers

• ECMAScript 6 (ES6)/ ECMAScript 2015 (ES2015): The 6th edition of ECMAScript, standardized in 2015. This standard has been partially implemented in most modern browsers.

Here are some key differences between ES5 and ES6:

• Arrow functions & string interpolation: Consider:

const greetings = (name) => {
      return `hello ${name}`;
}

and even:

const greetings = name => `hello ${name}`;

• Const. Const works like a constant in other languages in many ways but there are some caveats. Const stands for ‘constant reference’ to a value. So with const, you can actually mutate the properties of an object being referenced by the variable. You just can’t change the reference itself.

const NAMES = [];
NAMES.push("Jim");
console.log(NAMES.length === 1); // true
NAMES = ["Steve", "John"]; // error

• Block-scoped variables. The new ES6 keyword let allows developers to scope variables at the block level. Let doesn’t hoist in the same way var does.
• Default parameter values Default parameters allow us to initialize functions with default values. A default is used when an argument is either omitted or undefined — meaning null is a valid value.

// Basic syntax
function multiply (a, b = 2) {
     return a * b;
}
multiply(5); // 10

• Class Definition and Inheritance
  
  ES6 introduces language support for classes (class keyword), constructors (constructor keyword), and the extend keyword for inheritance.

• for-of operator
  
  The for...of statement creates a loop iterating over iterable objects.

• Spread Operator
  For objects merging
  

const obj1 = { a: 1, b: 2 }
const obj2 = { a: 2, c: 3, d: 4}
const obj3 = {...obj1, ...obj2}

• Promises Promises provide a mechanism to handle the results and errors from asynchronous operations. You can accomplish the same thing with callbacks, but promises provide improved readability via method chaining and succinct error handling.

const isGreater = (a, b) => {
  return new Promise ((resolve, reject) => {
    if(a > b) {
      resolve(true)
    } else {
      reject(false)
    }
    })
}
isGreater(1, 2)
  .then(result => {
    console.log('greater')
  })
 .catch(result => {
    console.log('smaller')
 })

• Modules exporting & importing Consider module exporting:

const myModule = { x: 1, y: () => { console.log('This is ES5') }}
export default myModule;

and importing:

import myModule from './myModule';

🔗 Source: FullStack.Cafe

Q2: What is IIFEs (Immediately Invoked Function Expressions)?

Topic: JavaScript
Difficulty: ⭐⭐⭐

It’s an Immediately-Invoked Function Expression, or IIFE for short. It executes immediately after it’s created:

(function IIFE(){
    console.log( "Hello!" );
})();
// "Hello!"

This pattern is often used when trying to avoid polluting the global namespace, because all the variables used inside the IIFE (like in any other normal function) are not visible outside its scope.

🔗 Source: stackoverflow.com

Q3: When should I use Arrow functions in ES6?

Topic: JavaScript
Difficulty: ⭐⭐⭐

I'm now using the following rule of thumb for functions in ES6 and beyond:

• Use function in the global scope and for Object.prototype properties.
• Use class for object constructors.
• Use => everywhere else.

Why use arrow functions almost everywhere?

• Scope safety: When arrow functions are used consistently, everything is guaranteed to use the same thisObject as the root. If even a single standard function callback is mixed in with a bunch of arrow functions there's a chance the scope will become messed up.
• Compactness: Arrow functions are easier to read and write. (This may seem opinionated so I will give a few examples further on).
• Clarity: When almost everything is an arrow function, any regular function immediately sticks out for defining the scope. A developer can always look up the next-higher function statement to see what the thisObject is.

🔗 Source: stackoverflow.com

Q4: What is the motivation for bringing Symbols to ES6?

Topic: JavaScript
Difficulty: ⭐⭐⭐

Symbols are a new, special kind of object that can be used as a unique property name in objects. Using Symbol instead of string's allows different modules to create properties that don't conflict with one another. Symbols can also be made private, so that their properties can't be accessed by anyone who doesn't already have direct access to the Symbol.

Symbols are a new primitive. Just like the number, string, and boolean primitives, Symbol have a function which can be used to create them. Unlike the other primitives, Symbols do not have a literal syntax (e.g how string have '') - the only way to create them is with the Symbol constructor in the following way:

let symbol = Symbol();

In reality, Symbol's are just a slightly different way to attach properties to an object - you could easily provide the well-known Symbols as standard methods, just like Object.prototype.hasOwnProperty which appears in everything that inherits from Object.

🔗 Source: stackoverflow.com

Q5: What are the benefits of using spread syntax in ES6 and how is it different from rest syntax?

Topic: JavaScript
Difficulty: ⭐⭐⭐

ES6's spread syntax is very useful when coding in a functional paradigm as we can easily create copies of arrays or objects without resorting to Object.create, slice, or a library function. This language feature is used often in Redux and rx.js projects.

function putDookieInAnyArray(arr) {
  return [...arr, 'dookie'];
}

const result = putDookieInAnyArray(['I', 'really', "don't", 'like']); // ["I", "really", "don't", "like", "dookie"]

const person = {
  name: 'Todd',
  age: 29,
};

const copyOfTodd = { ...person };

ES6's rest syntax offers a shorthand for including an arbitrary number of arguments to be passed to a function. It is like an inverse of the spread syntax, taking data and stuffing it into an array rather than unpacking an array of data, and it works in function arguments, as well as in array and object destructuring assignments.

function addFiveToABunchOfNumbers(...numbers) {
  return numbers.map(x => x + 5);
}

const result = addFiveToABunchOfNumbers(4, 5, 6, 7, 8, 9, 10); // [9, 10, 11, 12, 13, 14, 15]

const [a, b, ...rest] = [1, 2, 3, 4]; // a: 1, b: 2, rest: [3, 4]

const { e, f, ...others } = {
  e: 1,
  f: 2,
  g: 3,
  h: 4,
}; // e: 1, f: 2, others: { g: 3, h: 4 }

🔗 Source: github.com/yangshun

Q6: What are the differences between ES6 class and ES5 function constructors?

Topic: JavaScript
Difficulty: ⭐⭐⭐

Let's first look at example of each:

// ES5 Function Constructor
function Person(name) {
  this.name = name;
}

// ES6 Class
class Person {
  constructor(name) {
    this.name = name;
  }
}

For simple constructors, they look pretty similar.

The main difference in the constructor comes when using inheritance. If we want to create a Student class that subclasses Person and add a studentId field, this is what we have to do in addition to the above.

// ES5 Function Constructor
function Student(name, studentId) {
  // Call constructor of superclass to initialize superclass-derived members.
  Person.call(this, name);

  // Initialize subclass's own members.
  this.studentId = studentId;
}

Student.prototype = Object.create(Person.prototype);
Student.prototype.constructor = Student;

// ES6 Class
class Student extends Person {
  constructor(name, studentId) {
    super(name);
    this.studentId = studentId;
  }
}

It's much more verbose to use inheritance in ES5 and the ES6 version is easier to understand and remember.

🔗 Source: github.com/yangshun

Q7: What's the difference between .call and .apply?

Topic: JavaScript
Difficulty: ⭐⭐⭐

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

🔗 Source: github.com/yangshun

Q8: Why should we use ES6 classes?

Topic: JavaScript
Difficulty: ⭐⭐⭐

Some reasons you might choose to use Classes:

• The syntax is simpler and less error-prone.
• It's much easier (and again, less error-prone) to set up inheritance hierarchies using the new syntax than with the old.
• class defends you from the common error of failing to use new with the constructor function (by having the constructor throw an exception if this isn't a valid object for the constructor).
• Calling the parent prototype's version of a method is much simpler with the new syntax than the old (super.method() instead of ParentConstructor.prototype.method.call(this) or Object.getPrototypeOf(Object.getPrototypeOf(this)).method.call(this)).

Consider:

// **ES5**
var Person = function(first, last) {
    if (!(this instanceof Person)) {
        throw new Error("Person is a constructor function, use new with it");
    }
    this.first = first;
    this.last = last;
};

Person.prototype.personMethod = function() {
    return "Result from personMethod: this.first = " + this.first + ", this.last = " + this.last;
};

var Employee = function(first, last, position) {
    if (!(this instanceof Employee)) {
        throw new Error("Employee is a constructor function, use new with it");
    }
    Person.call(this, first, last);
    this.position = position;
};
Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.prototype.personMethod = function() {
    var result = Person.prototype.personMethod.call(this);
    return result + ", this.position = " + this.position;
};
Employee.prototype.employeeMethod = function() {
    // ...
};

And the same with ES6 classes:

// ***ES2015+**
class Person {
    constructor(first, last) {
        this.first = first;
        this.last = last;
    }

    personMethod() {
        // ...
    }
}

class Employee extends Person {
    constructor(first, last, position) {
        super(first, last);
        this.position = position;
    }

    employeeMethod() {
        // ...
    }
}

🔗 Source: stackoverflow.com

Q9: What is the preferred syntax for defining enums in JavaScript?

Topic: JavaScript
Difficulty: ⭐⭐⭐

Since 1.8.5 it's possible to seal and freeze the object, so define the above as:

var DaysEnum = Object.freeze({
    "monday": 1,
    "tuesday": 2,
    "wednesday": 3,
    ...
})

or

var DaysEnum = {
    "monday": 1,
    "tuesday": 2,
    "wednesday": 3,
    ...
}
Object.freeze(DaysEnum)

and voila! JS enums.

However, this doesn't prevent you from assigning an undesired value to a variable, which is often the main goal of enums:

let day = DaysEnum.tuesday
day = 298832342 // goes through without any errors

🔗 Source: stackoverflow.com

Q10: Explain the difference between Object.freeze() vs const

Topic: JavaScript
Difficulty: ⭐⭐⭐

const and Object.freeze are two completely different things.

• const applies to bindings ("variables"). It creates an immutable binding, i.e. you cannot assign a new value to the binding.

const person = {
    name: "Leonardo"
};
let animal = {
    species: "snake"
};
person = animal; // ERROR "person" is read-only

• Object.freeze works on values, and more specifically, object values. It makes an object immutable, i.e. you cannot change its properties.

let person = {
    name: "Leonardo"
};
let animal = {
    species: "snake"
};
Object.freeze(person);
person.name = "Lima"; //TypeError: Cannot assign to read only property 'name' of object
console.log(person);

🔗 Source: stackoverflow.com

Q11: What is Hoisting in JavaScript?

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐

Hoisting is the JavaScript interpreter's action of moving all variable and function declarations to the top of the current scope. There are two types of hoisting:

• variable hoisting - rare
• function hoisting - more common

Wherever a var (or function declaration) appears inside a scope, that declaration is taken to belong to the entire scope and accessible everywhere throughout.

var a = 2;
foo();                 // works because `foo()`
                         // declaration is "hoisted"

function foo() {
    a = 3;
    console.log( a );   // 3
    var a;             // declaration is "hoisted"
                         // to the top of `foo()`
}

console.log( a );   // 2

🔗 Source: FullStack.Cafe

Q12: Explain the Prototype Design Pattern

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐

The Prototype Pattern creates new objects, but rather than creating non-initialized objects it returns objects that are initialized with values it copied from a prototype - or sample - object. The Prototype pattern is also referred to as the Properties pattern.

An example of where the Prototype pattern is useful is the initialization of business objects with values that match the default values in the database. The prototype object holds the default values that are copied over into a newly created business object.

Classical languages rarely use the Prototype pattern, but JavaScript being a prototypal language uses this pattern in the construction of new objects and their prototypes.

🔗 Source: dofactory.com

Q13: What is the Temporal Dead Zone in ES6?

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐

In ES6 let and const are hoisted (like var, class and function), but there is a period between entering scope and being declared where they cannot be accessed. This period is the temporal dead zone (TDZ).

Consider:

//console.log(aLet)  // would throw ReferenceError

let aLet;
console.log(aLet); // undefined
aLet = 10;
console.log(aLet); // 10

In this example the TDZ ends when aLet is declared, rather than assigned.

🔗 Source: github.com/ajzawawi

Q14: When should you NOT use arrow functions in ES6? Name three or more cases.

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐

Arrow functions should NOT be used:

• When we want function hoisting - as arrow functions are anonymous.
• When we want to use this/arguments in a function - as arrow functions do not have this/arguments of their own, they depend upon their outer context.
• When we want to use named function - as arrow functions are anonymous.
• When we want to use function as a constructor - as arrow functions do not have their own this.
• When we want to add function as a property in object literal and use object in it - as we can not access this (which should be object itself).

🔗 Source: stackoverflow.com

Q15: What are the actual uses of ES6 WeakMap?

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐

WeakMaps provide a way to extend objects from the outside without interfering with garbage collection. Whenever you want to extend an object but can't because it is sealed - or from an external source - a WeakMap can be applied.

WeakMap is only available for ES6 and above. A WeakMap is a collection of key and value pairs where the key must be an object.

var map = new WeakMap();
var pavloHero = {
    first: "Pavlo",
    last: "Hero"
};
var gabrielFranco = {
    first: "Gabriel",
    last: "Franco"
};
map.set(pavloHero, "This is Hero");
map.set(gabrielFranco, "This is Franco");
console.log(map.get(pavloHero)); //This is Hero

The interesting aspect of the WeakMaps is the fact that it holds a weak reference to the key inside the map. A weak reference means that if the object is destroyed, the garbage collector will remove the entire entry from the WeakMap, thus freeing up memory.

🔗 Source: stackoverflow.com

Q16: Explain why the following doesn't work as an IIFE. What needs to be changed to properly make it an IIFE?

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐

function foo(){ }();

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 brackets) 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 brackets: (function foo(){ })() and (function foo(){ }()). 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

🔗 Source: github.com/yangshun

Q17: Could you compare usage of Module Pattern vs Constructor/Prototype pattern?

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐

The module pattern is typically used for namespacing, where you'll have a single instance acting as a store to group related functions and objects. This is a different use case from what prototyping is good for. They're not really competing with each other; you can quite happily use both together (eg put a constructor-function inside a module and say new MyNamespace.MyModule.MyClass(arguments)).

Constructor-functions and prototypes are one of the reasonable ways to implement classes and instances. They don't quite correspond to that model so you typically need to choose a particular scheme or helper method to implement classes in terms of prototypes.

🔗 Source: stackoverflow.com

Q18: What's the difference between ES6 Map and WeakMap?

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐⭐

They both behave differently when a object referenced by their keys/values gets deleted. Lets take the below example code:

var map = new Map();
var weakmap = new WeakMap();

(function() {
    var a = {
        x: 12
    };
    var b = {
        y: 12
    };

    map.set(a, 1);
    weakmap.set(b, 2);
})()

The above IIFE is executed there is no way we can reference {x: 12} and {y: 12} anymore. Garbage collector goes ahead and deletes the key b pointer from “WeakMap” and also removes {y: 12} from memory. But in case of “Map”, the garbage collector doesn’t remove a pointer from “Map” and also doesn’t remove {x: 12} from memory.

WeakMap allows garbage collector to do its task but not Map. With manually written maps, the array of keys would keep references to key objects, preventing them from being garbage collected. In native WeakMaps, references to key objects are held "weakly", which means that they do not prevent garbage collection in case there would be no other reference to the object.

🔗 Source: stackoverflow.com

Q19: Can you give an example of a curry function and why this syntax offers an advantage?

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐⭐

Currying is a pattern where a function with more than one parameter is broken into multiple functions that, when called in series, will accumulate all of the required parameters one at a time. This technique can be useful for making code written in a functional style easier to read and compose. It's important to note that for a function to be curried, it needs to start out as one function, then broken out into a sequence of functions that each accepts one parameter.

function curry(fn) {
  if (fn.length === 0) {
    return fn;
  }

  function _curried(depth, args) {
    return function(newArgument) {
      if (depth - 1 === 0) {
        return fn(...args, newArgument);
      }
      return _curried(depth - 1, [...args, newArgument]);
    };
  }

  return _curried(fn.length, []);
}

function add(a, b) {
  return a + b;
}

var curriedAdd = curry(add);
var addFive = curriedAdd(5);

var result = [0, 1, 2, 3, 4, 5].map(addFive); // [5, 6, 7, 8, 9, 10]

🔗 Source: github.com/yangshun

Q20: How to "deep-freeze" object in JavaScript?

Topic: JavaScript
Difficulty: ⭐⭐⭐⭐⭐

If you want make sure the object is deep frozen you have to create a recursive function to freeze each property which is of type object:

Without deep freeze:

let person = {
    name: "Leonardo",
    profession: {
        name: "developer"
    }
};
Object.freeze(person); // make object immutable
person.profession.name = "doctor";
console.log(person); //output { name: 'Leonardo', profession: { name: 'doctor' } }

With deep freeze:

function deepFreeze(object) {
    let propNames = Object.getOwnPropertyNames(object);
    for (let name of propNames) {
        let value = object[name];
        object[name] = value && typeof value === "object" ?
            deepFreeze(value) : value;
    }
    return Object.freeze(object);
}
let person = {
    name: "Leonardo",
    profession: {
        name: "developer"
    }
};
deepFreeze(person);
person.profession.name = "doctor"; // TypeError: Cannot assign to read only property 'name' of object

🔗 Source: medium.com

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