isNil not supporting type guards for undefined? | NestJS | Page 1

remote shale Dec 16, 2023, 6:07 PM

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hi, I have a question about 'isNil' function inside commons/utils/shared.utils.ts

Expected behavior

Its type guard must contain checks for both undefined and null values.

Current status:

The source code contains guards for both,
but the compiled d.ts file contains only the null type guard.

Proof:

https://github.com/nestjs/nest/blob/v10.2.10/packages/common/utils/shared.utils.ts

When I see this code, it contains

export const isNil = (val: any): val is null | undefined =>
  isUndefined(val) || val === null;

However, if I see the compiled version of it, it somehow weirdly erases the 'undefined' part of the type guard.

export declare const isNil: (val: any) => val is null;

You can check this out here https://www.npmjs.com/package/@nestjs/common/v/10.2.10?activeTab=code
and go to common/utils/shared.utils.d.ts

Why is this happening?

old willow Dec 16, 2023, 9:03 PM

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this is due to strictNullChecks: false at https://github.com/nestjs/nest/blob/8bba897216a1629ffc8e7029645f99fc9644a938/packages/tsconfig.build.json#L19C6-L19C22

GitHub

nest/packages/tsconfig.build.json at 8bba897216a1629ffc8e7029645f99...

A progressive Node.js framework for building efficient, scalable, and enterprise-grade server-side applications with TypeScript/JavaScript 🚀 - nestjs/nest

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btw such helpers are intended to be used internally, they are not part of the public API
So don't use it

steel remnant Dec 17, 2023, 8:50 AM

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there is a simple vanilla js work around you can use

const someVar = null
// works with: null undefined and empty strings ""
if(!someVar) {
// error
}

old willow Dec 17, 2023, 4:39 PM

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steel remnant there is a simple vanilla js work around you can use ```ts const someVar = null ...

I´d do someVar == undefined
it's the same as checking for both undefined and null at same time

magic sapphireBOT Dec 17, 2023, 4:39 PM

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This post has been marked as resolved. :white_check_mark:
Please read through the conversation and resolution if you are having the same issue, and then re-open the post if you are still having trouble, providing as much extra information as possible.

steel remnant Dec 17, 2023, 5:13 PM

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old willow I´d do `someVar == undefined` it's the same as checking for both undefined and...

But does empty string also work?

old willow Dec 17, 2023, 5:22 PM

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steel remnant But does empty string also work?

Nope, it will behave just like that isNil
Empty strings aren’t nils

steel remnant Dec 17, 2023, 5:24 PM

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old willow Nope, it will behave just like that isNil Empty strings aren’t nils

Fair point, but this is why my method is better i argue