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Arrays

Updated Dec 21, 2020 ·

Overview​

Arrays are used to store multiple values in a single variable. Instead of creating multiple variables for different values, we can bundle them into an array.

const friends = ["Michael", "Steven", "Peter"];
console.log(friends);

Output:

["Michael", "Steven", "Peter"]

Another way to create an array is using the new Array() syntax:

const years = new Array(1991, 1984, 2008, 2020);
console.log(years);

Output:

[1991, 1984, 2008, 2020]`

Accessing Array Elements​

Arrays are zero-based, meaning the first element is at index 0.

const friends = ["Michael", "Steven", "Peter", "Ted"];
console.log(friends[0]); // "Michael"
console.log(friends[2]); // "Peter"

To get the number of elements in the list:

console.log(friends.length) ;     // 4

We can also get the last element dynamically:

console.log(friends[friends.length - 1]); // "Ted"

Modifying an Array​

Arrays can be modified even when declared with const.

const friends = ["Michael", "Steven", "Peter", "Ted"];
friends[2] = "Jay";
console.log(friends);

Output:

["Michael", "Steven", "Jay, "Ted""]

However, we cannot reassign or override the entire array with this method:

friends = ["Bob", "Alice"]; // ❌ Error: Assignment to constant variable

Arrays with Different Data Types​

An array can hold different types of values:

const person = ["James", "Smith", 2025 - 1991, "Architect"];
console.log(person);

Output:

["James", "Smith", 34, "Architect"]

Arrays can also contain variables and another array.

const person = ["James", "Smith", 25, "Architect"];     // first Array
const organization = 'ABC Holdings';                    // variable
const platform = ["Singapore", organization, person]
console.log(platform);

Output:

Adding Elements​

Methods for adding elements to an array.

  • push() – Adds an element to the end of the array.
  • unshift() – Adds an element to the beginning of the array.

Example:

Add "Jay" to the end of the list:

const friends = ["Michael", "Steven", "Peter"];
friends.push("Jay");  
console.log(friends); // ["Michael", "Steven", "Peter", "Jay"]

Add "John" to the end of the list:

friends.unshift("John");  
console.log(friends); // ["John", "Michael", "Steven", "Peter", "Jay"]

Removing Elements​

Methods for removing elements from an array.

  • pop() – Removes the last element.
  • shift() – Removes the first element.

Example:

const friends = ["Michael", "Steven", "Peter", "Jay"] 

friends.pop();  
console.log(friends); // ["John", "Michael", "Steven", "Peter"]

friends.shift();  
console.log(friends); // ["Michael", "Steven", "Peter"]

Finding Elements​

Methods to check for elements in an array.

  • indexOf()

    – Returns the position of an element.

    • Returns -1 if the element is not found.

  • includes() – Returns true if the element exists, false otherwise.

    • Checks for strict equality (no type coercion).

Example:

console.log(friends.indexOf("Steven"));   // 1
console.log(friends.indexOf("Bob"));      // -1

console.log(friends.includes("Steven"));  // true
console.log(friends.includes("Bob"));     // false

Checking for string versus number:

friends.push(23);                         
console.log(friends);                     // ['Michael', 'Steven', 'Peter', 23]
console.log(friends.includes("23"));      // false (string vs number)
console.log(friends.includes(23));        // true

Using includes() in Conditionals​

The includes() method is useful for writing conditions.

const friends = ["Michael", "Steven", "Peter", "Jay"] 

if (friends.includes("Peter")) {
  console.log("You have a friend called Peter.");
}