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Project: endless Fibonacci

JavaScript · Unit 16: Iterators and generators

Fibonacci starts at 0 and 1, and from there each number is the sum of the two before it: 0, 1, 1, 2, 3, 5, 8… The sequence never ends, which is exactly what a generator is good at.

This is the machine that produces it: two variables shuffling along.

let a = 0;
let b = 1;
for (let i = 0; i < 5; i++) {
  console.log(a);
  const s = a + b;
  a = b;
  b = s;
}

Prints

0
1
1
2
3

Exercises in this lesson

You do them in the app, which checks them on the spot and explains why.

  1. 1. Complete the code

    Fill in the blanks so the generator hands over 0, 1, 1 and 2.

  2. 2. Write the code

    Write the fib() generator: it hands over the Fibonacci numbers forever, starting at 0 and 1. The firstN(n) function below is ready and keeps the first n of them.

  3. 3. Write the code

    Now write fibBelow(max): it returns an array with the Fibonacci numbers below max, in order. If none of them are, it returns an empty array. The fib() generator is already written.

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