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Declaring arrays

Learn to declare and initialize arrays with [].

Declaring arrays

To hold several values in a single variable you use an array. In JavaScript an array is declared with comma-separated values inside square brackets []: const colors = ["red", "blue", "green"];.

Each value in an array is called an element. An array may hold zero, one or more elements. An empty array is created with []; you can add elements later with push. Strings, numbers, booleans and even objects can all live in the same array; but good practice keeps elements of a similar type.

The length property

The .length property tells how many elements an array has. [10, 20, 30].length evaluates to 3. length is a property, not a method; there is no parenthesis at the end.

length is especially useful for loop bounds: with for (let i = 0; i < arr.length; i++) you walk all elements. An empty array has length 0. Although length can be set directly, it is uncommon; to resize an array you use methods like push, pop or splice.

Practice Declare and print the array

Declare a variable colors and set its value to ["red", "blue", "green"]. Then print colors.length with console.log. The output on a single line must be 3.

Starter code

index.js
// Declare colors and print its length

Sample solution

index.js
const colors = ["red", "blue", "green"];
console.log(colors.length);

Expected output

3

Questions and answers

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

1 Which is a valid array declaration?

index.js
const arr = ...;
  • const arr = [1, 2, 3]; Correct answer
  • const arr = {1, 2, 3};
  • const arr = (1, 2, 3);
  • const arr = <1, 2, 3>;

Explanation: Arrays are declared inside square brackets [].

2 Write the array literal

Question 
const colors = ____;
Answer 
const colors = ["red", "blue"];

Explanation: An array is written with square brackets.

3 Match each expression to its meaning

Term Meaning
[] Empty array
[1, 2, 3] Three-element array
.length Element count
["a"] Single-element array

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