TypeScript: checking Map keys and Array indices

JavaScript has two common patterns:

Maps: We check the existence of a key via .has() before retrieving the associated value via .get().
Arrays: We check the length of an Array before performing an indexed access.

These patterns don’t work as well in …


This content originally appeared on 2ality – JavaScript and more and was authored by Dr. Axel Rauschmayer

JavaScript has two common patterns:

  • Maps: We check the existence of a key via .has() before retrieving the associated value via .get().
  • Arrays: We check the length of an Array before performing an indexed access.

These patterns don’t work as well in TypeScript. This blog post explains why and presents alternatives.


This content originally appeared on 2ality – JavaScript and more and was authored by Dr. Axel Rauschmayer


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Dr. Axel Rauschmayer | Sciencx (2025-06-21T00:00:00+00:00) TypeScript: checking Map keys and Array indices. Retrieved from https://www.scien.cx/2025/06/21/typescript-checking-map-keys-and-array-indices/

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