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Arrow Functions - JavaScript Interview Questions

JavaScript Fundamentals: Arrow Functions

What is the definition of an arrow function?​

View Answer:
Interview Response: An arrow function expression is a compact alternative to a traditional function expression, but it is limited, and we should not use it in all situations.

Technical Response: An arrow function is a concise syntax for defining anonymous functions in JavaScript, using the arrow notation. It offers shorter syntax, lexical scoping of "this", and can't be used as a constructor.

Code Example:

// Arrow Function
let sayHello = (name) => 'Hello, ' + name;
console.log(sayHello('JavaScript!'));

// Function Expression
let sayHello = function (name) {
  return 'Hello, ' + name;
};

console.log(sayHello('JavaScript!'));

How do arrow functions differ from function expressions?​

View Answer:
Interview Response: Arrow functions provide a shorter syntax, don't have their own this, arguments, super, or new.target, and can't be used as constructors, unlike function expressions.

Technical Response:

Differences & Limitations:

  1. It does not have its binding to this or super and should not get used as a method.
  2. It does not have arguments or new.target keywords.
  3. Not suitable for the call, apply and bind methods, which generally rely on establishing a scope.
  4. It cannot get used as a constructor.
  5. It cannot use yield within its body.

Code Example:

'use strict';

var obj = {
  // does not create a new scope
  i: 10,
  b: () => console.log(this.i, this),
  c: function () {
    console.log(this.i, this);
  },
};

obj.b(); // prints undefined, Window {...} (or the global object)
obj.c(); // prints 10, Object {...}

If there are no arguments in an arrow function. Do you need to add the parentheses in the function signature?​

View Answer:
Interview Response: Yes, if an arrow function has no arguments, you need to include empty parentheses in the function signature to indicate the absence of parameters. This syntax is required for proper arrow function definition. Otherwise, it will throw a syntax error.

Code Example:

// Arrow Function with no argument
let sayHi = () => console.log('Hello!');

sayHi(); // returns Hello!

What is the difference between a regular function and an arrow function?​

View Answer:
Interview Response: The key difference between regular and arrow functions in JavaScript is that arrow functions have a concise syntax and do not bind to their own "this" context, while regular functions do.

Code Example:

 let language = {
  name: "JavaScript",
  javascript1:() => {
   console.log(`Hello, ${this.name}!`); // no 'this' binding here
  },
  javascript2(){
   console.log(`I love ${this.name}!`); // 'this' binding works here
  }
 };
 language.javascript1(); // Hello, undefined!
 language.javascript2(); // I love JavaScript!

Can you dynamically create a function with an arrow function?​

View Answer:
Interview Response: Yes, it is possible to create an arrow function in JavaScript dynamically. An example is a ternary statement that returns two anonymous arrow functions.

Code Example:

let age = prompt('What is your age?', 18);

let welcome = age < 18 ? () => console.log('Hello') : () => console.log('Greetings!');

welcome();

What is the difference between a single and multiline statement in an arrow function?​

View Answer:
Interview Response: In an arrow function, a single-line statement is implicitly returned while a multi-line statement requires an explicit "return" statement.

Code Example:

let sum = (a, b) => {
  // the curly brace opens a multiline function
  let result = a + b;
  return result; // if we use curly braces, then we need an explicit "return”.
};

console.log(sum(1, 2)); //

// Single Line
let sum = (a, b) => a + b;
console.log(sum(3, 6)); // returns 9

What are the advantages of using arrow functions?​

View Answer:
Interview Response: The advantages of using arrow functions in JavaScript include shorter syntax, implicit return, and lexical ‘this’ binding.

How is the 'this' keyword treated in arrow functions?​

View Answer:
Interview Response: In arrow functions, the 'this' keyword is lexically bound to the surrounding scope, rather than having its own 'this' value.

What does lexical 'this' binding mean in arrow functions?​

View Answer:
Interview Response: "Lexical this" binding in arrow functions means they don't create their own 'this' context; instead, they inherit 'this' from their surrounding, enclosing scope, reducing common 'this'-related issues.

How does 'this' binding differ in arrow functions?​

View Answer:
Interview Response: Arrow functions bind the “this” keyword lexically to the context where they are defined, instead of dynamically like regular functions.

Can you pass default parameters to an arrow function?​

View Answer:
Interview Response: Yes, you can use the same syntax as regular functions to pass default parameters to an arrow function.

Code Example:

const greet = (name = 'friend') => {
  console.log(`Hello, ${name}!`);
};

greet(); // Output: Hello, friend!
greet('John'); // Output: Hello, John!

Can arrow functions be used as constructors?​

View Answer:
Interview Response: No, arrow functions cannot be used as constructors because they don't have their own ‘this’ value.

Code Example:

const Person = (name) => {
  this.name = name;
};

const john = new Person('John'); // Throws an error: Person is not a constructor

Can arrow functions use rest parameters?​

View Answer:
Interview Response: Yes, in JavaScript, arrow functions can use rest parameters. Rest parameters are denoted with three dots (…) before the parameter name and gather remaining arguments into an array, allowing for a more flexible function.

Code Example:

const sum = (...numbers) => {
  let total = 0;
  for (let number of numbers) {
    total += number;
  }
  return total;
};

console.log(sum(1, 2, 3)); // Output: 6
console.log(sum(4, 5, 6, 7)); // Output: 22

What is an implicit return in arrow functions?​

View Answer:
Interview Response: An implicit return in an arrow function occurs when the function consists of a single expression without curly braces, automatically returning the expression's result without needing a return statement.

Code Example:

const multiply = (a, b) => a * b;

console.log(multiply(2, 3)); // Output: 6
console.log(multiply(4, 5)); // Output: 20

How do you create a higher-order function using arrow functions?​

View Answer:
Interview Response: We can define a function that takes one or more functions as arguments or returns a function. Arrow functions can be used as higher-order functions in the same way as regular functions.

Code Example:

const withGreeting = (greeting) => (name) => {
  console.log(`${greeting}, ${name}!`);
};

const greetHello = withGreeting('Hello');
const greetHi = withGreeting('Hi');

greetHello('John'); // Output: Hello, John!
greetHi('Jane'); // Output: Hi, Jane!

Can arrow functions be used as methods in an object?​

View Answer:
Interview Response: Yes, arrow functions can be used as methods in an object. However, they should be used with caution as the ‘this’ keyword will not refer to the object itself but to the surrounding scope.

What are the common use cases for arrow functions?​

View Answer:
Interview Response: Arrow functions are commonly used for Object methods, event listeners, callbacks, and other functions that require shorter, more concise syntax.

Code Example:

Concise anonymous functions:

setTimeout(() => {
  console.log('Delayed execution');
}, 1000);

Callback functions:

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map((num) => num * 2);
console.log(doubled); // Output: [2, 4, 6, 8, 10]

Lexical 'this' binding in React components:

class MyComponent extends React.Component {
  handleClick = () => {
    // Function logic using 'this' to access component's state and props
  };

  render() {
    return <button onClick={this.handleClick}>Click me</button>;
  }
}

In these examples, arrow functions are used for concise anonymous functions, as callback functions for array methods like map, and to maintain the lexical 'this' binding in React components for accessing component state and props.