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Create a JavaScript generator function based on a condition

Almost every generator function needs a termination condition. Instead of writing the same code over and over again, we can easily create a generator function that takes a predicate function as an argument and keeps producing new values as long as the condition is met.

Given a condition, we can initialize the current value using a provided seed value. Then, using a while loop we can iterate as long as the predicate function called with the current val returns true.

Inside the loop, we can use yield to return the current val and optionally receive a new seed value, nextSeed. Finally, we can use the next function to calculate the next value from the current val and the nextSeed.

const generateWhile = function* (seed, predicate, next) {
  let val = seed;
  let nextSeed = null;
  while (predicate(val)) {
    nextSeed = yield val;
    val = next(val, nextSeed);
  return val;

[...generateWhile(1, v => v <= 5, v => ++v)]; // [1, 2, 3, 4, 5]
💡 Tip

You can switch this code to work until a condition is met by simply changing the while loop condition to !predicate(val).

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