Typescript: function isNaN only takes number as its argument { isNaN(number: number): boolean) }, instead of accepting argument with `any` as its type.

Created on 26 Oct 2019 · 5Comments · Source: microsoft/TypeScript

Expected behavior:
The function isNaN(param) (short-term for _Not a Number_) returns a boolean which indicates if the said param literal type is a number (return false) or not (return true). Hence, the function should accept a param with any types whatsoever, and it should be isNaN(param: any): boolean.

Actual behavior:
The function is static-typed to isNaN(number: **number**): boolean, which prevents parameter other than with a number type (and defeats the purpose of the function), leaving the only option to add //@ts-ignore to the line before it and which in turns, disables typescript on that line.

Tested on 3.6.3 and 3.7.0-dev.20191018

Playground Link:
http://www.typescriptlang.org/play/?ts=3.7-Beta#code/MYewdgzgLgBGCuBbARgUwE4wLwwIwCYBmAKFEljBCgEEA5JNTHAcgGFqAlZ44gSwloBDWgAoEKDAEoYAehkxgAC1TAA1hBgh4UADQx0qKPHRgYAM0EAbCKgB0PfkNGUa9CemlyYl3mCi+AcxgMdBB0PQB3RV4lGH4YCNRedAATBXAIXhSMQJgoaI143igNAAdjUpAbWyA

Related Issues:
None that I found so far

Source

hello-wahyu

All 5 comments

Related Issues: None that I found so far

Just use the term isNaN for a search to find plenty of duplicates.

#3947
#4002
#11414
#12302
#13227
#15173
#16427
#19153
#21065
#23464
#29607
#34609

MartinJohns on 26 Oct 2019

👀1 👍1

The intended purpose of that function is to tell you if the value is specifically the reserved value NaN; non-number arguments are first coerced to number which is generally not what people expect. For example isNaN("8foo") is false but isNaN("foo8") is true because number coercion turns the latter into NaN.

fatcerberus on 26 Oct 2019

👍1

Related Issues: None that I found so far

Just use the term isNaN for a search to find plenty of duplicates.

#3947

#4002

#11414

#12302

#13227

#15173

#16427

#19153

#21065

#23464

#29607

#34609

Hmm, I did do a search with 'isNaN' and 'NaN' myself, apparently there's a problem with my browser since it didn't show it. Thanks to pointing it out!

hello-wahyu on 26 Oct 2019

The intended purpose of that function is to tell you if the value is _specifically the reserved value NaN_; non-number arguments are first coerced to number which is generally not what people expect. For example isNaN("8foo") is false but isNaN("foo8") is true because number coercion turns the latter into NaN.

Ah, I see, this does makes much sense then and I didn't really comprehend it before (even though I do know JS will try its 'best' to coerce when dealing with 'truthy' and 'falsy' values).

I'm gonna close this issue then. Thank you so much for all of your help!

hello-wahyu on 26 Oct 2019

👍1