10 Must-Know JavaScript Coding Interview Questions

As a JavaScript developer, it's essential to be prepared for common interview questions that test your skills and knowledge. Here are 10 must-know questions, along with detailed answers and code examples, to help you ace your next interview.

1. Debounce

Debouncing is a crucial technique used to manage repetitive or frequent events, particularly in the context of user input, such as keyboard typing or resizing a browser window. The primary goal of debouncing is to improve performance and efficiency by reducing the number of times a particular function or event handler is triggered; the handler is only triggered when the input has stopped changing.

Example Usage:

The debounce function from Lodash can be used to create a debounced version of a function, as shown below:

import { debounce } from 'lodash';

const searchInput = document.getElementById('search-input');

const debouncedSearch = debounce(() => {
  // Perform the search operation here
  console.log('Searching for:', searchInput.value);
}, 300);

searchInput.addEventListener('input', debouncedSearch);

Key Aspects of Debouncing

Delays the execution of a function until a certain amount of time has passed since the last input event
Helps optimize performance by preventing unnecessary computations or network requests during rapid user input
Can be implemented in different versions, including:
- Leading version: Invokes the callback at the start of the timeout
- Trailing version: Invokes the callback at the end of the timeout
- Maximum delay: The maximum time the callback is allowed to be delayed before it is invoked

Relationship with Throttling

Debouncing and Throttling are related techniques, but they serve different purposes. Throttling is a technique that limits the frequency of a function's execution, while debouncing delays the execution of a function until a certain amount of time has passed since the last input event.

Practice implementing a Debounce function on GreatFrontEnd

2. `Promise.all`

Promise.all() is a key feature in JavaScript that simplifies handling multiple asynchronous operations concurrently, particularly when there are dependencies among them. It accepts an array of promises and returns a new promise that resolves to an array of results once all input promises have resolved, or rejects if any input promise rejects.

Being proficient with Promise.all() demonstrates a front-end engineer's capability to manage complex asynchronous workflows efficiently and handle errors effectively, which is crucial for their daily tasks.

const promise1 = fetch('https://api.example.com/data/1');
const promise2 = fetch('https://api.example.com/data/2');
const promise3 = fetch('https://api.example.com/data/3');

Promise.all([promise1, promise2, promise3])
  .then((responses) => {
    // This callback runs only when all promises in the array have resolved.
    console.log('All responses:', responses);
  })
  .catch((error) => {
    // Handle any errors from any promise.
    console.error('Error:', error);
  });

In this example, Promise.all() is used to fetch data from three different URLs concurrently. The .then() block executes only when all three promises resolve. If any promise rejects, the .catch() block handles the error.

This is a valuable topic for front-end interviews since candidates are often tested on their knowledge of asynchronous programming and their ability to implement polyfills. Promise.all() has related functions like Promise.race() and Promise.any(), which can also be covered in interviews, making it a versatile topic to master.

Practice implementing Promise.all() on GreatFrontEnd

3. Deep Equal

Deep Equal is an essential concept in JavaScript for comparing two objects or arrays to determine if they are structurally identical. Unlike shallow equality, which checks if the references of the objects are the same, deep equality checks if the values within the objects or arrays are equal, including nested structures.

Here's a basic implementation of a deep equal function in JavaScript:

function deepEqual(obj1, obj2) {
  if (obj1 === obj2) return true;

  if (
    obj1 == null ||
    typeof obj1 !== 'object' ||
    obj2 == null ||
    typeof obj2 !== 'object'
  )
    return false;

  let keys1 = Object.keys(obj1);
  let keys2 = Object.keys(obj2);

  if (keys1.length !== keys2.length) return false;

  for (let key of keys1) {
    if (!keys2.includes(key) || !deepEqual(obj1[key], obj2[key])) return false;
  }

  return true;
}

// Example usage
const object1 = {
  name: 'John',
  age: 30,
  address: {
    city: 'New York',
    zip: '10001',
  },
};

const object2 = {
  name: 'John',
  age: 30,
  address: {
    city: 'New York',
    zip: '10001',
  },
};

console.log(deepEqual(object1, object2)); // true

In this example, the deepEqual function recursively checks if two objects (or arrays) are equal. It first checks if the two objects are the same reference. If not, it verifies that both are objects and not null. Then, it compares the keys and values recursively to ensure all nested structures are equal.

This topic is valuable for front-end interviews as it tests a candidate's understanding of deep vs. shallow comparisons, recursion, and handling complex data structures.

Practice implementing Deep Equal on GreatFrontEnd

4. Event Emitter

An EventEmitter class in JavaScript is a mechanism that allows objects to subscribe to, listen for, and emit events when specific actions or conditions are met. This class supports the observer pattern, where an object (the event emitter) keeps a list of dependents (observers) and notifies them of any changes or events. The EventEmitter is also part of the Node.js API.

// Example usage
const eventEmitter = new EventEmitter();

// Subscribe to an event
eventEmitter.on('customEvent', (data) => {
  console.log('Event emitted with data:', data);
});

// Emit the event
eventEmitter.emit('customEvent', { message: 'Hello, world!' });

Creating an EventEmitter class requires an understanding of object-oriented programming, closures, the this keyword, and basic data structures and algorithms. Follow-up questions in interviews might include implementing an API for unsubscribing from events.

Practice implementing an Event Emitter on GreatFrontEnd

5. `Array.prototype.reduce()`

Array.prototype.reduce() is a built-in method in JavaScript that allows you to apply a function against an accumulator and each element in the array (from left to right) to reduce it to a single value. This method is highly versatile and can be used for a variety of tasks such as summing numbers, flattening arrays, or grouping objects.

// Example: Summing numbers in an array
const numbers = [1, 2, 3, 4, 5];
const sum = numbers.reduce(function (accumulator, currentValue) {
  return accumulator + currentValue;
}, 0);

console.log(sum); // Output: 15

Array.prototype.reduce() is a frequently asked topic in front-end interviews, especially by major tech companies, alongside its sister methods, Array.prototype.map(), Array.prototype.filter(), and Array.prototype.concat(). Modern front-end development often utilizes functional programming style APIs like Array.prototype.reduce(), making it an excellent opportunity for candidates to demonstrate their knowledge of prototypes and polyfills. Although it seems straightforward, there are several deeper aspects to consider:

Do you know how to use both the initial and returned values of the accumulator?
How does the method handle sparse arrays?
Are you familiar with the four parameters accepted by the reduce callback function?
What happens if the array is mutated during the reduction process?
Can you implement a polyfill for Array.prototype.reduce()?

Practice implementing the Array.prototype.reduce() function on GreatFrontEnd

6. Flatten

In JavaScript, "flattening" refers to the process of converting a nested array into a single-level array. This is useful for simplifying data structures and making them easier to work with. JavaScript provides several ways to flatten arrays, with the most modern and convenient method being the Array.prototype.flat() method introduced in ES2019.

// Example: Flattening a nested array
const nestedArray = [1, [2, [3, [4, [5]]]]];
const flatArray = nestedArray.flat(Infinity);

console.log(flatArray); // Output: [1, 2, 3, 4, 5]

In this example, the flat() method is used with a depth of Infinity to completely flatten the deeply nested array into a single-level array. The flat() method can take a depth argument to specify the level of flattening if the array is not deeply nested.

Before ES2019, flattening arrays required custom implementations or the use of libraries like Lodash. Here’s a basic custom implementation using recursion:

// Custom implementation of flattening an array
function flattenArray(arr) {
  return arr.reduce((acc, val) => {
    return Array.isArray(val) ? acc.concat(flattenArray(val)) : acc.concat(val);
  }, []);
}

const nestedArray = [1, [2, [3, [4, [5]]]]];
const flatArray = flattenArray(nestedArray);

console.log(flatArray); // Output: [1, 2, 3, 4, 5]

This custom flattenArray function uses the reduce() method to concatenate values into a single array, recursively flattening any nested arrays encountered.

Practice implementing Flatten function on GreatFrontEnd

7. Data Merging

Data merging in JavaScript involves combining multiple objects or arrays into a single cohesive structure. This is often necessary when dealing with complex data sets or integrating data from different sources. JavaScript provides several methods to merge data, including the spread operator, Object.assign(), and various array methods.

Merging Objects

Using the Spread Operator

The spread operator (...) is a concise way to merge objects. It creates a new object by copying the properties from the source objects.

const obj1 = { a: 1, b: 2 };
const obj2 = { b: 3, c: 4 };

const mergedObj = { ...obj1, ...obj2 };

console.log(mergedObj); // Output: { a: 1, b: 3, c: 4 }

In this example, obj2's b property overwrites obj1's b property in the merged object.

Using `Object.assign()`

Object.assign() is another method to merge objects. It copies all enumerable properties from one or more source objects to a target object.

const obj1 = { a: 1, b: 2 };
const obj2 = { b: 3, c: 4 };

const mergedObj = Object.assign({}, obj1, obj2);

console.log(mergedObj); // Output: { a: 1, b: 3, c: 4 }

Merging Arrays

Using the Spread Operator

The spread operator can also merge arrays by concatenating them.

const array1 = [1, 2, 3];
const array2 = [4, 5, 6];

const mergedArray = [...array1, ...array2];

console.log(mergedArray); // Output: [1, 2, 3, 4, 5, 6]

Using Array.concat()

The concat() method merges two or more arrays into a new array.

const array1 = [1, 2, 3];
const array2 = [4, 5, 6];

const mergedArray = array1.concat(array2);

console.log(mergedArray); // Output: [1, 2, 3, 4, 5, 6]

Deep Merging

For deep merging, where nested objects and arrays need to be merged, a custom function or a library like Lodash can be used. Here's a simple custom implementation:

function deepMerge(target, source) {
  for (const key in source) {
    if (source[key] instanceof Object && key in target) {
      Object.assign(source[key], deepMerge(target[key], source[key]));
    }
  }
  Object.assign(target || {}, source);
  return target;
}

const obj1 = { a: 1, b: { x: 10, y: 20 } };
const obj2 = { b: { y: 30, z: 40 }, c: 3 };

const mergedObj = deepMerge(obj1, obj2);

console.log(mergedObj); // Output: { a: 1, b: { x: 10, y: 30, z: 40 }, c: 3 }

Using Lodash `merge`

Lodash is a popular utility library in JavaScript that provides many helpful functions, including merge. The _.merge function in Lodash recursively merges properties of the source objects into the destination object, which is particularly useful for deep merging of nested objects.

const _ = require('lodash');

const obj1 = { a: 1, b: { x: 10, y: 20 } };
const obj2 = { b: { y: 30, z: 40 }, c: 3 };

const mergedObj = _.merge({}, obj1, obj2);

console.log(mergedObj); // Output: { a: 1, b: { x: 10, y: 30, z: 40 }, c: 3 }

In this example, _.merge deep merges obj1 and obj2, ensuring that nested properties are combined correctly.

Practice implementing Data Merging function on GreatFrontEnd

8. `getElementsByClassName`

In JavaScript, getElementsByClassName is a method used to select elements from the DOM (Document Object Model) based on their CSS class names. It returns a live HTMLCollection of elements that match the specified class name(s).

Basic Usage

You can use getElementsByClassName by calling it on the document object and passing one or more class names as arguments:

// Select all elements with the class name "example"
const elements = document.getElementsByClassName('example');

// Loop through the selected elements
for (let i = 0; i < elements.length; i++) {
  console.log(elements[i].textContent);
}

Multiple Class Names

You can specify multiple class names separated by spaces:

const elements = document.getElementsByClassName('class1 class2');

This will select elements that have both class1 and class2.

Live `HTMLCollection`

The HTMLCollection returned by getElementsByClassName is live, meaning it updates automatically when the DOM changes. If elements with the specified class name are added or removed, the collection is updated accordingly.

Alternative Methods

`querySelectorAll`

For more complex selections based on CSS selectors, including class names, IDs, attributes, etc., querySelectorAll provides more flexibility:

const elements = document.querySelectorAll('.example');

Practice implementing getElementsByClassName on GreatFrontEnd

9. Memoize

Memoization is a technique used in programming to optimize expensive function calls by caching their results. In JavaScript, memoization involves storing the results of expensive function calls and returning the cached result when the same inputs occur again.

The basic idea behind memoization is to improve performance by avoiding redundant calculations. Here’s a simple example of memoization in JavaScript:

function expensiveOperation(n) {
  console.log('Calculating for', n);
  return n * 2;
}

// Memoization function
function memoize(func) {
  const cache = {};

  return function (n) {
    if (cache[n] !== undefined) {
      console.log('From cache for', n);
      return cache[n];
    } else {
      const result = func(n);
      cache[n] = result;
      return result;
    }
  };
}

const memoizedExpensiveOperation = memoize(expensiveOperation);

console.log(memoizedExpensiveOperation(5)); // Output: Calculating for 5, 10
console.log(memoizedExpensiveOperation(5)); // Output: From cache for 5, 10
console.log(memoizedExpensiveOperation(10)); // Output: Calculating for 10, 20
console.log(memoizedExpensiveOperation(10)); // Output: From cache for 10, 20

How Memoization Works

Caching Results: The memoize function wraps around expensiveOperation and maintains a cache object.
Cache Check: Before executing expensiveOperation, memoize checks if the result for a given input (n) is already stored in the cache.
Returning Cached Result: If the result is found in the cache, memoize returns it directly without re-executing expensiveOperation.
Storing Result: If the result is not in the cache, memoize computes it by calling expensiveOperation(n), stores the result in the cache, and then returns it.

In modern JavaScript, libraries like Lodash provide utilities for memoization, making it easier to apply this optimization technique across different functions and use cases.

Practice implementing Memoize function on GreatFrontEnd

10. Get

Before the introduction of optional chaining (?.) in JavaScript, accessing nested properties in an object could lead to errors if any part of the path did not exist.

For example:

const user = {
  name: 'John',
  address: {
    street: '123 Main St',
  },
};

const city = user.address.city; // throws an error because address.city is undefined

Using `get` from Lodash

To avoid this, developers used workarounds like the get function from Lodash to access nested properties within objects with ease:

const user = {
  name: 'John',
  address: {
    city: 'New York',
  },
};

console.log(_.get(user, 'address.city')); // 'New York'
console.log(_.get(user, 'address.street')); // 'undefined'

Here, _.get retrieves the value located at obj.user.address.city, handling potential undefined values gracefully.

However, with the introduction of optional chaining (?.) in JavaScript, we can now access nested properties in a safer way:

const user = {
  name: 'John',
  address: {
    street: '123 Main St',
  },
};

const city = user.address?.city; // returns undefined instead of throwing an error

The ?. operator allows us to access properties in a way that stops evaluating the expression if any part of the path is null or undefined, preventing errors and returning undefined instead.

Practice implementing get function on GreatFrontEnd

Conclusion

These questions cover essential concepts in JavaScript, and understanding them will help you tackle more complex problems in your interviews. Remember to practice and be ready to explain your thought process and code examples!

1. Debounce

Key Aspects of Debouncing

Relationship with Throttling

2. `Promise.all`

3. Deep Equal

4. Event Emitter

5. `Array.prototype.reduce()`

6. Flatten

7. Data Merging

Merging Objects

Using the Spread Operator

Using `Object.assign()`

Merging Arrays

Using the Spread Operator

Using Array.concat()

Deep Merging

Using Lodash `merge`

8. `getElementsByClassName`

Basic Usage

Multiple Class Names

Live `HTMLCollection`

Alternative Methods

`querySelectorAll`

9. Memoize

How Memoization Works

10. Get

Using `get` from Lodash

Conclusion

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10 Must-Know JavaScript Coding Interview Questions

1. Debounce

Key Aspects of Debouncing

Relationship with Throttling

2. Promise.all

3. Deep Equal

4. Event Emitter

5. Array.prototype.reduce()

6. Flatten

7. Data Merging

Merging Objects

Using the Spread Operator

Using Object.assign()

Merging Arrays

Using the Spread Operator

Using Array.concat()

Deep Merging

Using Lodash merge

8. getElementsByClassName

Basic Usage

Multiple Class Names

Live HTMLCollection

Alternative Methods

querySelectorAll

9. Memoize

How Memoization Works

10. Get

Using get from Lodash

Conclusion

2. `Promise.all`

5. `Array.prototype.reduce()`

Using `Object.assign()`

Using Lodash `merge`

8. `getElementsByClassName`

Live `HTMLCollection`

`querySelectorAll`

Using `get` from Lodash