`String.prototype.split()` with a regex containing a capturing group can return an array of `string | undefined`.

⚙ Compilation target

any

⚙ Library

esnext, dom

Missing / Incorrect Definition

Using split with a negative lookahead and a capturing group can return undefined. See the attached screenshot. When iterating over the array, we lose type safety and encounter runtime errors.

Sample Code

'a,b,c'.split(/,(?!(a))/) // Types as string[]

Documentation Link

No response

Start at the String.prototype.split entry point in the esnext and dom library definitions and compare its current return type with the sample 'a,b,c'.split(/,(?!(a))/). Check how capturing groups that do not participate are represented at runtime, then add coverage showing that the resulting array type accounts for undefined.