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isDefined ​

Problem ​

You need to filter out undefined values from an array or narrow a type that includes undefined.

Solution ​

Use isDefined(value) to narrow away undefined. Works directly as an Array.filter callback.

ts
import { isDefined } from '@vielzeug/arsenal';

const values: (number | undefined)[] = [1, undefined, 2, undefined, 3];
const numbers: number[] = values.filter(isDefined);
// [1, 2, 3]

Pitfalls ​

  • Allows null through — use isNil to exclude both null and undefined.