🐛 Bug Report

This bug is typescript-only. The getWorksheet method specifies Worksheet as its return type, but when passing a non-existing sheet name as parameter, it returns in reality undefined.

Lib version: 4.3.0

Why is it an issue

If the undefined possibility isn't specified, then it's easy for developers to forget to account for this possibility when parsing a file coming from a user.

Steps To Reproduce

const wb = new ExcelJS.Workbook();
const ws = wb.getWorksheet('nonExisting');
// type of ws : Worksheet
// actual value of ws : undefined
const row = ws.getRow(1) // throws

The expected behaviour:

the return type should be Worksheet | undefined

Possible solution

// modify the return type in index.d.ts
// line 1746
getWorksheet(indexOrName: number | string): Worksheet | undefined;