Nested Arrays

An element of an array can be any value, even another array. An array inside an array is called a nested array. Nested arrays are useful for data that comes in groups, such as study groups:

const studyGroups = [["Jennifer", "Boris"], ["David", "Ava", "Nancy"]];
console.log(studyGroups);

This prints:

[["Jennifer", "Boris"], ["David", "Ava", "Nancy"]]

The array studyGroups has two elements, and each of them is an array of names: one study group. The first index selects a group, which is an ordinary array:

console.log(studyGroups[1]);

This prints:

["David", "Ava", "Nancy"]

To get one student, add a second index in square brackets. studyGroups[1][0] first takes the element at index 1 (the second group, ["David", "Ava", "Nancy"]) and then, from that array, the element at index 0:

console.log(studyGroups[1][0]);

This prints:

David

Everything you know about arrays works for the inner arrays too. studyGroups.length is 2, the number of groups, while studyGroups[1].length is 3, the number of students in the second group. And studyGroups[0].push("Tim") would add Tim to the end of the first group.