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Nullish coalescing

Learn nullish coalescing with ?? and optional chaining.

Nullish coalescing

The nullish coalescing operator ?? returns the right side if the left side is null or undefined; otherwise it returns the left side.

For example, userInput ?? 'guest'. If userInput is null, the result is 'guest'; otherwise it is the value of userInput.

This is ideal for assigning default values. The difference from ||: || also catches falsy values (0, '', false); ?? only catches null/undefined.

Optional chaining

The optional chaining operator is written with ?. and provides safe access to an object's property. If the property does not exist, it returns undefined instead of throwing an error.

For example, user?.name. If user is null or undefined, the result is undefined; no error is thrown.

This is very useful for nested objects: order?.customer?.address?.city. If any intermediate step is missing, the chain stops safely.

Practice Default value with ??

Declare userInput = null and defaultValue = 'guest'. Compute name as userInput ?? defaultValue. Call console.log(name).

Starter code

index.js
const userInput = null;
const defaultValue = "guest";
// Compute name as userInput ?? defaultValue
// Log it with console.log

Sample solution

index.js
const userInput = null;
const defaultValue = "guest";
const name = userInput ?? defaultValue;
console.log(name);

Expected output

guest

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match Nullish vs OR

Term Meaning
?? Nullish coalescing
|| Logical or
?. Optional chaining
&& Logical and

2 What does the ?? operator do?

  • Divides two numbers
  • Returns the right side if the left is null/undefined Correct answer
  • Returns the right side if the left is false
  • Concatenates two strings

Explanation: ?? returns the right side if the left is null/undefined.

3 ?? example

Question 
const name = userInput ____ 'guest';
Answer 
const name = userInput ?? 'guest';

Explanation: ?? returns the right side if the left is null/undefined.

All lessons in Beginner

1 Node.js basics

  1. What is Node.js?
  2. Your first program
  3. ES modules
  4. package.json
  5. CommonJS vs ESM
  6. Summary - Node.js basics

2 Variables

  1. let and const
  2. var and scope
  3. Type inference
  4. Destructuring
  5. Spread and rest
  6. Summary - Variables

3 Data types

  1. Primitive types
  2. Template literals
  3. Number and Math
  4. Symbol
  5. typeof
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. process.argv
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Nullish coalescing
  6. Summary - Operators

6 Strings

  1. String methods
  2. String search
  3. substring and slice
  4. replace and split
  5. padStart and trim
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Ternary
  5. Optional chaining
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. for...of
  4. for...in
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. push and pop
  4. shift and unshift
  5. splice and slice
  6. Summary - Arrays

11 Array methods

  1. map
  2. filter
  3. reduce
  4. find and some
  5. forEach
  6. Summary - Array methods

12 Objects

  1. Object literal
  2. Property access
  3. Computed properties
  4. Object methods
  5. Object spread
  6. Summary - Objects

13 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Default parameters
  5. Rest parameters
  6. Summary - Functions

14 Callbacks

  1. What are callbacks?
  2. Higher-order functions
  3. forEach with callbacks
  4. Error-first callbacks
  5. Callback hell
  6. Summary - Callbacks

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Promises

  1. What are promises?
  2. then and catch
  3. Promise.all
  4. Promise.race
  5. finally
  6. Summary - Promises

17 async/await

  1. async function
  2. await
  3. try/catch
  4. await with Promise.all
  5. Sequential await
  6. Summary - async/await

18 fs module

  1. fs.promises
  2. readFile
  3. writeFile
  4. appendFile
  5. existsSync
  6. Summary - fs module

19 path module

  1. path.join
  2. path.resolve
  3. path.parse
  4. path.dirname and basename
  5. path.extname
  6. Summary - path module

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. fs persistence
  5. Putting it together