22 Important JavaScript/ES6/ES2015 Interview Questions

• 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';

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);

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 }

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.

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.

Generators are functions which can be exited and later re-entered. Their context (variable bindings) will be saved across re-entrances. Generator functions are written using the function* syntax. When called initially, generator functions do not execute any of their code, instead returning a type of iterator called a Generator. When a value is consumed by calling the generator's next method, the Generator function executes until it encounters the yield keyword.

The function can be called as many times as desired and returns a new Generator each time, however each Generator may only be iterated once.

function* makeRangeIterator(start = 0, end = Infinity, step = 1) {
    let iterationCount = 0;
    for (let i = start; i < end; i += step) {
        iterationCount++;
        yield i;
    }
    return iterationCount;
}

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.

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

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

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.

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() {
        // ...
    }
}