String.split is defined as returning string[], but values in the array can be undefined.

[fpintos]

Bug Report

🔎 Search Terms

string.split undefined

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about string.split

⏯ Playground Link

Playground Link

💻 Code

const [a,b] = "hello".split('@');
console.log([a,b]); // b is typed as string, but it can be undefined. a, on the other hand, is guaranteed to be string and not undefined.

// If b is passed to a function that strictly expects valid string, we get a runtime error.
// Compiler should have prevented it by detecting that b can be string|undefined.
console.log(foo(a,b));
function foo(a:string, b:string) {
    return a.length + b.length;
}

🙁 Actual behavior

Variable b is typed as string, when it can have an undefined value. Correct type would be string | undefined, so compiler can detect if it is being incorrectly passed to a function that strictly expects a valid string value.

🙂 Expected behavior

String.split() return type should be [string, ...(string | undefined)[]] to be correct.
This would be breaking change for programs using strict checks, but it will help catch unexpected undefined values being passed around.

Current workaround is to fix the type on every split() call:

const [a,b] = "hello".split('@') as [string, ...(string | undefined)[]];

The compiler will then flag invalid calls using b as string.
Playground Link

[whzx5byb]

String.split() return type should be [string, ...(string | undefined)[]] to be correct.

That's not true because ''.split('') returns [].

And if you want to include undefined type you can enable noUncheckedIndexedAccess.

[MartinJohns]

Duplicate of #49635. Used search terms: split in:title

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.