Infer type guard => array.filter(x => !!x) should refine Array<T|null> to Array<T>

TypeScript Version: 2.3

Code

const evenSquares: number[] =
    [1, 2, 3, 4]
        .map(x => x % 2 === 0 ? x * x : null)
        .filter(x => !!x);

with strictNullChecks enabled.

Expected behavior:

This should type check. The type of evenSquares should be number[].

Actual behavior:

The type of evenSquares is deduced as (number|null)[], despite the null values being removed via the call to .filter.

[robertmassaioli]

To avoid everybody having to write the same helper functions over and over again I bundled isPresent, isDefined and isFilled into a helper library:

When Typescript bundles this functionality in I’ll remove the package.

To use the library:

import { isPresent, isDefined, isFilled } from 'ts-is-present';

arrayWithUndefinedAndNullValues.filter(isPresent)
arrayWithUndefinedValues.filter(isDefined)
arrayWithNullValues.filter(isFilled)

To see why this package works please read the readme of the NPM package.

[RyanCavanaugh]

Reopening to track this since both other issues now target lib.d.ts changes