Implement flatMap

Very HardHigher Order FunctionsCustom ImplementationsPRO

Practice higher-order JavaScript by implementing flatMap behavior without calling the native helper. This very hard exercise makes the transform-then-flatten decision explicit, including callback metadata, one-level flattening, non-array returns, and stable output order.

What you'll practice

Work with higher-order JavaScript by implementing flatMap behavior without calling the native helper
The transform-then-flatten decision explicit
Callback metadata
One-level flattening
Non-array returns
Stable output order

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