TypeScript reports incorrect error

[johndeighan]

🔎 Search Terms

type narrowing

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type narrowing

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250410#code/EQVwzgpgBGAuBOBLAxrYBuAUMg9gOzigBsARAQ1jIC4oAKPEAWwCMJ4oAfGBRPAc05QQeACYQAZrwgiAlAG0AulAC8UOZgCQAFgBMAGk0ByMs2SGDG4WMl5pmhZmz5CRAEo4A7jThJ+ilcTklAB0jGQADrS0iLAQjDIqAHxQAN6aiOJ0MXEqyqpWElKyUPAQsCDweFCGhlgaGXSwAJ7hEDiZ2Yy5qoYMLGyGCaXllVCdwbA4AGKIAB7StDoydQ20za3tY7FdeT0+vHyDJWUVVZ1YAL4yjrgEOEQQwSKI8LRuntdAA

💻 Code

"use strict";
const lData: (number | string | undefined)[] = [
	42,
	'abc',
	undefined
	]

const lRow: string[] = lData.map((item) => {
	if (item === undefined) return '';
	if (typeof item === 'number') return item.toFixed(2);
	if (typeof item === 'string') return item;
})

console.dir(lRow)```


### 🙁 Actual behavior

The above code results in errors, even though the mapping function being used can never return an undefined value. Oddly, if I use a switch statement instead of a series of if statements, no errors are reported:

"use strict";
const lData: (number | string | undefined)[] = [
42,
'abc',
undefined
]

const lRow: string[] = lData.map((item) => {
switch(typeof item) {
case 'undefined': {
return '';
}
case 'number': {
return item.toFixed(2);
}
case 'string': {
return item;
}
}
})

console.dir(lRow)

### 🙂 Expected behavior

There should be no errors in either case since the items being mapped over can only be undefined, a string or a number. Note that in both of the examples, there is no "else" case.

### Additional information about the issue

_No response_

[nmain]

Duplicate of #21985. My search terms were if never in:title. It works if you explicitly annotate the never:

const lRow: string[] = lData.map((item) => {
	if (item === undefined) return '';
	if (typeof item === 'number') return item.toFixed(2);
	if (typeof item === 'string') return item;
	item satisfies never;
	return item;
})

[typescript-bot]

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

[johndeighan]

Nice to have a hacky work-around, though I think it would be better to actually fix the problem. Meanwhile, I'll just use a switch statement since it doesn't have this problem.