In JavaScript, an Array is an ordered list of values. You write one with square brackets: ["red", "green", "blue"]. Each value is an element. The first element is at index 0.
length is a property, so you read it without parentheses: colors.length. Methods do the work of changing or copying the list. push() adds to the same array. map() and filter() return a new array and leave the original alone. Array.isArray(value) is the check for “is this a list?”, because typeof [] is "object".
This page teaches what an Array is. The Array Methods index and Array Properties index list every method and property guide (push(), map(), length, and more).
colors[0] is the first element. colors.at(-1) is the last. colors[colors.length] is undefined.
Common ways to create and read an array:
const colors = ["red", "green", "blue"];
const first = colors[0]; // "red"
const count = colors.length; // 3
const last = colors.at(-1); // "blue"
const copy = colors.slice(); // new array, same items
const ok = Array.isArray(colors); // trueIdeas every JavaScript Array tutorial builds on:
| Idea | Example | Remember |
|---|---|---|
| Literal | ["red", "green"] | Ordered list of any values |
| Index | colors[0] | First item is index 0 |
| length | colors.length | A property, not a method |
| Add | colors.push("gold") | Changes that same array |
| Copy | nums.map(...) | Returns a new array |
| Check | Array.isArray(x) | typeof [] is "object" |
Three short programs that show how Array works in a JS tutorial.
The first element is 0. at(-1) counts from the end.
const colors = ["red", "green", "blue"];
console.log(colors[0]);
console.log(colors.length);
console.log(colors.at(-1));"red", the first element.3 because there are three elements."blue", the last element.Keep the doubled list in a new variable. push edits nums.
const nums = [1, 2, 3];
const doubled = nums.map(function (n) {
return n * 2;
});
nums.push(4);
console.log(nums.join(","));
console.log(doubled.join(","));[2, 4, 6] and does not edit nums.4 to the end of nums.filter() builds a new array. Array.isArray confirms it is a list.
const scores = [10, 40, 70];
const passed = scores.filter(function (n) {
return n >= 40;
});
console.log(passed.join(","));
console.log(Array.isArray(passed));
console.log(Array.isArray("40"));40 and 70. 10 fails the test.[10, 40, 70]. The result lives in passed.false. A string is not an array.After you know what an Array is, open the indexes for full tutorials on every essential Array API in this JavaScript series.
| Start with | Why |
|---|---|
push() | Add an item to the end |
map() | Build a new array from each item |
filter() | Keep only the items that pass a test |
find() | Return the first matching item |
Full lists: Array Methods index and Array Properties index.
Open the index for push(), map(), filter(), find(), and the rest.
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