Data Structures

Data Structures#

JavaScript’s built-in collection types cover most everyday needs.

Arrays#

Ordered, zero-indexed, dynamically sized.

const xs = [1, 2, 3];
xs.push(4);                // [1, 2, 3, 4]
xs.pop();                  // 4
xs.unshift(0);             // [0, 1, 2, 3]
const sub = xs.slice(1, 3);
const i   = xs.indexOf(2);

const doubled = xs.map((x) => x * 2);
const evens   = xs.filter((x) => x % 2 === 0);
const sum     = xs.reduce((a, b) => a + b, 0);

Objects#

Hash maps with string or symbol keys.

const user = { name: "operator", age: 36 };
user.role = "admin";
delete user.age;

for (const [k, v] of Object.entries(user)) {
  console.log(k, v);
}

const keys = Object.keys(user);
const merged = { ...user, active: true };

Map / Set#

When keys are non-string or insertion order matters, prefer Map over a plain object. Set stores unique values.

const m = new Map();
m.set("a", 1).set("b", 2);
m.get("a");       // 1
m.has("c");       // false
m.size;           // 2

const s = new Set([1, 2, 2, 3]);
s.add(4);
s.has(2);         // true
[...s];           // [1, 2, 3, 4]

Tuples#

JavaScript has no first-class tuple type. The convention is to use a fixed-shape array, often with destructuring at the call site.

function divmod(a, b) {
  return [Math.floor(a / b), a % b];
}

const [q, r] = divmod(17, 5);

Strings#

Immutable sequences of UTF-16 code units. They have many of the same methods as arrays.

const s = "hello world";
s.length;             // 11
s.toUpperCase();      // "HELLO WORLD"
s.split(" ");         // ["hello", "world"]
s.includes("world");  // true
s.replace("world", "operator");

Template literals interpolate expressions and span multiple lines.

const name = "operator";
const greeting = `hello,
${name}!`;

Typed Arrays#

For binary data and numeric workloads, use typed arrays.

const buf = new Uint8Array([72, 101, 108, 108, 111]);
const text = new TextDecoder().decode(buf);   // "Hello"

WeakMap / WeakSet#

Like Map / Set, but keys must be objects and are held weakly: they don’t prevent garbage collection. Useful for attaching metadata to objects without leaking memory.

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