Arrays and indexes

An array stores an ordered group of values. JavaScript arrays can mix value types, although keeping related values together usually makes code easier to understand. Arrays can grow and shrink with methods such as push() and pop().

Creating arrays

Use square brackets when the values are known. Array(n).fill(0) creates n slots and fills each one with zero.

let scores = [88, 91, 76, 95, 83];
let counts = Array(4).fill(0);
counts[0] = 12;
counts[1] = 9;

Array(4).fill(0) contains four zeroes because fill(0) explicitly supplies that value. Array(4) alone contains four empty slots.

Accessing array elements

Array indexes start at 0, so the first element is scores[0]. An array of length 5 has indexes 0 through 4. Reading scores[5] produces undefined; it does not add a new element.

let scores = [88, 91, 76, 95, 83];
console.log(scores[0]); // 88
console.log(scores[4]); // 83
scores[2] = 80;
console.log(scores[2]); // 80

Array length and loops

Arrays use the length property without parentheses. The last valid index is array.length - 1. Use an index loop when positions matter and for...of when only values matter.

for (let i = 0; i < scores.length; i++) {
    console.log(`scores[${i}] = ${scores[i]}`);
}

for (const score of scores) {
    console.log(score);
}

Common operations

You can rewrite elements, search with a loop, append with push(), and remove the last element with pop().

let values = [4, 0, 7, 0, 2, 9];
values[2] = 5;
for (const value of values) {
    console.log(value + " ");
}