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snippets/js/s/unfold.md

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+---
+title: Unfold array
+type: snippet
+language: javascript
+tags: [function,array]
+cover: dog-waiting
+dateModified: 2020-09-15T16:28:04+03:00
+---
+
+Builds an array, using an iterator function and an initial seed value.
+
+- Use a `while` loop and `Array.prototype.push()` to call the function repeatedly until it returns `false`.
+- The iterator function accepts one argument (`seed`) and must always return an array with two elements ([`value`, `nextSeed`]) or `false` to terminate.
+
+```js
+const unfold = (fn, seed) => {
+  let result = [],
+    val = [null, seed];
+  while ((val = fn(val[1]))) result.push(val[0]);
+  return result;
+};
+```
+
+```js
+var f = n => (n > 50 ? false : [-n, n + 10]);
+unfold(f, 10); // [-10, -20, -30, -40, -50]
+```