I do not understand your problem. I think your test cases are not good. You define property name as optional in case PATCH is in groups.

it('should fail on patch when name is undefined', async () => {
    const errors = await getValidationErrors({ name: undefined }, PATCH);
    expect(errors).not.toEqual([]);
  });
  it('should fail on patch when name is null', async () => {
    const errors = await getValidationErrors({ name: null }, PATCH);
    expect(errors).not.toEqual([]);
  });

So Test should succeed on patch when name is undefined/null.
From docs:

I use nestjs to validate a DTO from my controller

I have this

class MyDTO {
  @IsOptional()
  @IsNotEmpty()
  locale?: string
}

I send this body json

{
  "locale": null
}

The expected behaviour:

• ERROR 400, locale should not be empty

The actual behaviour:

• CODE 200, locale null is ignored.

If I have this DTO

class MyDTO {
  @IsNotEmpty()
  locale?: string
}

Then null value throws CODE 400 which is expected.

@IsOptional() should ignores only when the key is not passed in the request (aka. undefined) or an option to not ignores null value.

I understand now. But as it is a breaking change I want to have more feedback from community.

I don't know if is the same issue, but i need to validate if a variable is notEmpty, notNull and notUndefined. Is that possible?

@vlapo How about something like an extra config option? The implementation I've got in my fork doesn't break backwards compatibility (as the option defaults to true, which is the default behaviour):

@isOptional({ nullable: false }) // object.hasOwnProperty(property);
@isOptional({ nullable: true }) // value !== undefined && value !== null
@isOptional() // value !== undefined && value !== null

The commit on the fork that adds this behaviour: se-internal@5ac9fa0

We would love this as well, ether with a nullable flag, or as a separate set of decorators (although naming them can be difficult).
@sam3d, your fork looks good to me, would you mind creating a PR?

Thanks @fhp! Yeah we had originally tried to create a new decorator and must've spent over an hour trying to come up with a good name. Eventually, we decided it made more sense to borrow the design pattern from typeorm's @ManyToOne() decorator (a nullable property that defaults to true).

I'll definitely put it into a PR at some point in the next couple of days! Just waiting until I have a moment to add tests. Confirmation from @vlapo or another maintainer that at least the design proposal is accepted would be amazing so I know I'm not doing it for nothing 😄

Any update for this? really want to see this feature in next release
@sam3d

I agree about the strange behavior of this.

I like the API proposed by @sam3d. We can use the current behavior as default and add a warning in the changelog about eventually being flipped.

I will take a crack at this at the weekend.

@NoNameProvided Any update? 10 days has been passed

How about keeping @IsOptional as-is and adding new decorators @IsNullable and @IsUndefinable.
Any support for this?

For other parts of TypeScript, "optional" means either the property is not set, or it is set to undefined. In no other places does "optional" imply "nullable", whereas "nullable" itself can sometimes imply "optional" (c.f. NonNullable utility type). The naming of IsOptional is really confusing.

I suggest the following changes to align with the terminology of TypeScript:

1. @IsOptional() taking a new strict parameter, defaulting to false which allows null for back-compat, and true for denying null. The strict parameter should default to true at some later version and possibly throw if it is set to false.
2. @IsNullable with a strict parameter defaulting to true which only allows null, and false for also allowing undefined.

I think

@IsOptional({ nullable: false })

looks great, as suggested by @sam3d
@vlapo Are there any plans on implementing that?

Meanwhile, I suggest creating two separate decorators:

import { ValidateIf } from 'class-validator';

export function CanBeUndefined() {
  return ValidateIf((object, value) => value !== undefined);
}

export function CanBeNull() {
  return ValidateIf((object, value) => value !== null);
}

In case anyone is interested, I've had the exact same issue and implemented this (alongside quite a few more decorators) in class-validator-extended, specifically Optional() and Nullable().

This is still an issue, please can this be updated? Some fantastic solutions have been presented. I am happy to make a PR

any update on this one? And what about PartialType which is making all props optional? Can we somehow pass this nullable param to PartialType as well? 🤔