Function isPropertyNotNil

Predicate / type guard function, checks if the given property is neither null nor undefined. Narrows the type of the given property within the object type from T | null | undefined to just T.

Two variants:

A simple function for direct calls (e.g. in if-statement-conditions). It asks for the object to check as well as the property.

The second variant is a factory-function which only gets the property. It returns a pre-configured predicate function for usage with functional calls like Array#filter.

Example:

const x: { a: number, b: string | undefined } = { a: 1, b: 'example' };

// simple usage: pass object and property path
if (isPropertyNotNil(x, 'b')) {
  // type of x is now { a: number, b: string }
  x.b.charAt(3); // safe to access b
}

// array usage: just pass the property to check
[x]
  .filter(isPropertyNotNil('b'))
  // type is now [{ a: number, b: string }]
  .map(x => x.b.charAt(3)) // safe to access b
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