JavaScript Generators og Iterators: Komplett Guide 2026

JavaScript-generatorer (function*) er spesielle funksjoner som kan settes på pause og gjenopptas med yield.

Generator-grunnlag: function* og yield

En generatorfunksjon returnerer et generatorobjekt som implementerer Iterator-protokollen.

// Generator function syntax: function* with yield
function* counter(start = 0) {
    let i = start;
    while (true) {
        yield i++;    // pauses here, returns i, resumes on next()
    }
}

const gen = counter(1);
console.log(gen.next()); // { value: 1, done: false }
console.log(gen.next()); // { value: 2, done: false }
console.log(gen.next()); // { value: 3, done: false }

// Finite generator
function* range(start, end, step = 1) {
    for (let i = start; i < end; i += step) {
        yield i;
    }
}

// for...of automatically calls .next() and stops at done: true
for (const num of range(0, 10, 2)) {
    console.log(num); // 0, 2, 4, 6, 8
}

// Spread operator works with generators
const nums = [...range(1, 6)]; // [1, 2, 3, 4, 5]

// Destructuring works too
const [a, b, c] = range(10, 20); // a=10, b=11, c=12

Toveiskommunikasjon: yield som uttrykk

yield er ikke bare return — det er et uttrykk som mottar en verdi via next(value).

// Generators support two-way communication
// yield receives values via next(value)
function* calculator() {
    let result = 0;
    while (true) {
        const input = yield result;  // pauses and sends result; receives input
        if (input === null) break;
        result += input;
    }
    return result;
}

const calc = calculator();
calc.next();        // start: { value: 0, done: false }
calc.next(10);      // add 10: { value: 10, done: false }
calc.next(5);       // add 5: { value: 15, done: false }
calc.next(null);    // stop: { value: 15, done: true }

// Generator as stateful iterator
function* idGenerator(prefix = 'id') {
    let id = 1;
    while (true) {
        const reset = yield `${prefix}-${id}`;
        if (reset) {
            id = 1;
        } else {
            id++;
        }
    }
}

const ids = idGenerator('user');
console.log(ids.next().value);       // 'user-1'
console.log(ids.next().value);       // 'user-2'
console.log(ids.next(true).value);   // 'user-1' (reset)
console.log(ids.next().value);       // 'user-2'

yield*: delegere til andre iterables

yield* delegerer til et annet iterable.

// yield* — delegate to another iterable
function* innerGen() {
    yield 'a';
    yield 'b';
    yield 'c';
}

function* outerGen() {
    yield 1;
    yield* innerGen();    // delegate: yields 'a', 'b', 'c'
    yield* [4, 5, 6];    // works with any iterable
    yield 7;
}

console.log([...outerGen()]); // [1, 'a', 'b', 'c', 4, 5, 6, 7]

// Practical: flatten nested arrays
function* flatten(arr) {
    for (const item of arr) {
        if (Array.isArray(item)) {
            yield* flatten(item); // recursive delegation
        } else {
            yield item;
        }
    }
}

const nested = [1, [2, [3, 4], 5], [6, 7]];
console.log([...flatten(nested)]); // [1, 2, 3, 4, 5, 6, 7]

// Tree traversal with yield*
function* walkTree(node) {
    yield node.value;
    for (const child of node.children ?? []) {
        yield* walkTree(child); // depth-first traversal
    }
}

Async-generatorer: streaming av data

Async-generatorer kombinerer generatorer med async/await for streaming.

// Async generators: async function* with yield
async function* streamLines(url) {
    const response = await fetch(url);
    const reader = response.body.getReader();
    const decoder = new TextDecoder();
    let buffer = '';

    while (true) {
        const { done, value } = await reader.read();
        if (done) {
            if (buffer) yield buffer;
            break;
        }
        buffer += decoder.decode(value, { stream: true });
        const lines = buffer.split('\n');
        buffer = lines.pop() ?? '';
        for (const line of lines) {
            yield line;   // yield each complete line
        }
    }
}

// Usage with for await...of
async function processCSV(url) {
    let lineNumber = 0;
    for await (const line of streamLines(url)) {
        lineNumber++;
        if (lineNumber === 1) continue; // skip header
        const [name, score] = line.split(',');
        console.log(`${name}: ${score}`);
    }
}

// SSE (Server-Sent Events) as async generator
async function* sseStream(url) {
    const response = await fetch(url);
    const reader = response.body.getReader();
    const decoder = new TextDecoder();

    for await (const chunk of readChunks(reader)) {
        const text = decoder.decode(chunk);
        for (const line of text.split('\n')) {
            if (line.startsWith('data: ')) {
                yield JSON.parse(line.slice(6));
            }
        }
    }
}

Virkelige mønstre

Generatorer muliggjør late pipelines som bare behandler nødvendige data.

// Real-world: infinite scroll with generator
function* paginator(fetchPage) {
    let page = 1;
    let hasMore = true;

    while (hasMore) {
        const { items, totalPages } = yield fetchPage(page);
        hasMore = page < totalPages;
        page++;
    }
}

// Lazy pipeline with generators
function* map(iterable, fn) {
    for (const item of iterable) {
        yield fn(item);
    }
}

function* filter(iterable, predicate) {
    for (const item of iterable) {
        if (predicate(item)) yield item;
    }
}

function* take(iterable, n) {
    let count = 0;
    for (const item of iterable) {
        if (count++ >= n) break;
        yield item;
    }
}

// Lazy pipeline — no intermediate arrays created!
const first10EvenSquares = [
    ...take(
        filter(
            map(range(1, Infinity), x => x * x),
            x => x % 2 === 0
        ),
        10
    )
];
// [4, 16, 36, 64, 100, 144, 196, 256, 324, 400]

// Observable-like: cancelable async iteration
async function* withTimeout(asyncIterable, timeoutMs) {
    const timeout = setTimeout(() => {
        throw new Error('Stream timed out');
    }, timeoutMs);
    try {
        for await (const item of asyncIterable) {
            yield item;
        }
    } finally {
        clearTimeout(timeout);
    }
}

Generatorer vs alternativer

Beste praksis

• Generatorer for late sekvenser, async/await for enkeltoperasjoner.
• return for tidlig avslutning, try/finally for opprydding.
• Async-generatorer perfekte for HTTP-streaming, SSE, WebSocket.
• Lage gjenbrukbare pipeline-hjelpere (map, filter, take).
• TypeScript: annotere med Generator for typesikkerhet.

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