Array indices are possibly 'undefined'

[aminpaks]

TypeScript Version: 2.5.2

Code

function test(statement: string): string {
  const parts = statement.split('-'); // <- parts will have at least one element
  let firstElement = parts.shift(); // <- firstElement cannot be undefined at this point

  if (parts.length > 1) {
    let secondElement = parts.shift(); // <- Still says that secondElement may be undefined!
  }
}
test('first-second');

Expected behavior:

• String prototype split method always creates an array with at least one elements and therefore shift method cannot produce an undefined type.
• Once we check the length of an array shift, pop cannot produce undefined type.

Actual behavior:
shift, pop methods or accessing arrays indices return undefined even though the array has elements.

[DanielRosenwasser]

There's not an easy way to communicate the probing you're doing with the return type of the methods using the type system today. You can convince the type system you know what you're doing with the non-null assertion operator (i.e. the ! operator):

let secondElement = parts.shift()!;

[kitsonk]

Duplicate of #11117

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.