JS Built-ins
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ArrayArray.isArrayintermediate

Array.isArray() — filter only arrays

📖 Usage Example
const mixed = [1, 'a', [2, 3], {b: 4}, [5]]
mixed.filter(Array.isArray)  // → [[2, 3], [5]]

Pass Array.isArray as a filter callback to extract only the array elements from a mixed collection.

Array.isArray() — filtering a mixed collection

Array.isArray is often used as a predicate in .filter() to pull only array values out of a mixed collection.

Challenge: Implement keepOnlyArrays that takes an array of unknown values and returns only the elements that are themselves arrays.

keepOnlyArrays([1, [2], 'hi', [3, 4], null]) // → [[2], [3, 4]]
keepOnlyArrays([1, 2, 3])                    // → []
💡 Hints (2)
  • • `Array.isArray` is a function — you can pass it directly to `.filter()` without wrapping.
  • • The return type is `unknown[][]` because the kept elements are themselves arrays.