Arrays and the first item

The idea

An array holds an ordered collection inside square brackets. Commas separate its items. For const tasks = ["read", "code"], tasks[0] is "read" and tasks[1] is "code". An array with two items has length 2 but last index 1. An empty array has length 0 and no first item.

Assignment to a position changes that item: tasks[0] = "review" replaces the first value. tasks.push("test") appends at the end and increases the length. const prevents assigning a different array to tasks; it does not make the array contents immutable. Say whether your operation is allowed to change its input before using a mutating method.

Start by tracing one array change at a time. Later, a loop will visit its items and a function will accept an array as an argument. The collection and copying chapters build on this picture; you do not need callbacks or nested copying to use your first array.

const tasks = ["read", "code"];
tasks[0] = "review";
tasks.push("test");
console.log(tasks[0]);
console.log(tasks.length);
console.log(tasks[tasks.length - 1]);

review
3
test

Replacing an existing item leaves the length unchanged. Appending adds a third item at index 2.