Allow simple extension of custom validators/sanitizers for better readability and reusability

[ysfaran]

Is your feature request related to a problem? Please describe.

When I add a custom validator, which I need to reuse, I usually create a function like this:

export const isToken = (value) => {
  if (typeof value !== "string") {
    throw Error("Value must be a string");
  }

  if (!value.startsWith("0x")) {
    throw Error("Value  must start with '0x'");
  }

  if (value.length !== 42) {
    throw Error("Value must be 42 characters long");
  }

  return true;
};

Usage:

body("token").custom(isToken),

I use the snippet custom(isToken) above for a lot of validations and also reuse it for different middlewars like param instead of body. This unfortunatley breaks the cleanness of the fluent api and makes it harder to read. You also need to import isToken everywhere you use it. For customSanitizer it's quite similar.

Describe the solution you'd like

express-validator should allow to extend it's validators/sanitizers in an easier way, similiar to how jest does it with expect.extend.

A good solution would be to export new functions from the library:

import { extend } from "express-validator";

which will allow to add custom validators and sanitizers:

extend.validators({
  isToken,
});

extend.sanitizers({
  toSomeSanitizer: /*..*/,
});

Usage afterwards:

body("token").isToken().toSomeSanitizer(),

With this solution the API would look much cleaner and it would simplify reusability in contrast to custom or customSanitizer.

Implementation Idea

The feature could be implemented by extending the prototype of

express-validator/src/chain/validators-impl.ts

Line 8 in 690cd63

export class ValidatorsImpl<Chain> implements Validators<Chain> {

export const extend = {
  validators(customValidators){
    for([name, validator] of Object.entries(customValidators)){
      // note: currently this.custom doesn't seem to support options
      ValidatorsImpl.prototype[name] = function(options) { return this.custom(validator, options); }
    }
  }
}
  
  // sanitizers() ...
}

It should also be supported to pass options to custom validators, e.g.:

export const isToken = (value, { length = 42 } = {} ) => {
  // ...

  if (value.length !== length) {
    throw Error(`Value must be ${length} characters long`);
  }

  return true;
};

body("token").isToken({ length: 18 }),

(I just had a rough look into to the code and there might be better ways to implement it. Feel free to suggest more appropriate solutions.)

[fedeci]

I completely agree with you about this feature requirement, it is in the v7 roadmap right now, we just need to implement a solution😄

[ysfaran]

@fedeci great 😄 is the road map publicly visible somewhere?

[fedeci]

Yes #990

[gustavohenke]

This is now possible in v7.0.0!
Check out docs here