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Narrowing types by `typeof` inside `switch (true)` does not work

See original GitHub issue

TypeScript Version: 3.8.3 Nightly

Search Terms:

  • switch statement
  • switch (true)


let x: string | undefined;

if (typeof x !== 'undefined') {
  x.trim(); // works as expected

switch (true) {
  case typeof x !== 'undefined':
    x.trim(); // thinks x is potentially undefined

Expected behavior:

Narrowing types should work inside the case.

Actual behavior:

Any assertion in case seems to be disregarded

Playground Link:

Related Issues:

Issue Analytics

  • State:open
  • Created 4 years ago
  • Reactions:32
  • Comments:17 (6 by maintainers)

github_iconTop GitHub Comments

joschacommented, Mar 4, 2020

Two come to mind: • readability for big amounts of cases • being able to use fall-throughs for overlapping cases

Having said that I understand it is an edge case. I came across this pattern in a bigger codebase and wanted to make a change. That’s when I noticed that the types are not narrowed in each case.

polomskycommented, Apr 15, 2021

I have similar usecase:

switch (true) {
    case error instanceof CustomError1:
    case error instanceof CustomError2:
    case error instanceof Error:
        return String(error);

And I am using this syntax in javascript because it looks better than chain of if else. Sadly can not use in typescript

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