JS Practice — Built-in Objects

Array.prototype.flatMap() — duplicate each element

📖 Usage Example

const nums = [1, 2, 3]
nums.flatMap(n => [n, n])  // → [1, 1, 2, 2, 3, 3]

Use flatMap() to map each element to an array and flatten the results by one level.

Array.prototype.flatMap()

Array.prototype.flatMap(callback) maps each element using a callback and then flattens the result by one level. It is equivalent to arr.map(fn).flat(1) but slightly more efficient.

Challenge: Implement duplicateEach(arr) that returns a new array where each element appears twice, using flatMap(x => [x, x]).

duplicateEach([1, 2, 3]) // → [1, 1, 2, 2, 3, 3]
duplicateEach([])        // → []

💡 Hints (2)

• `flatMap(x => [x, x])` returns `[x, x]` for each element, which are then flattened into the result.
• The flattening is exactly one level deep — nested arrays inside the returned arrays are not unwrapped further.