Reactive forms conditional custom validation

Question

I'm using reactive form validation in angular, and I'm trying to add different messages for different
in-validities, for example if I have a score field that should be between 1-100, I want to have a message when the user enters a letter and a different message if he enters a number bigger than 100.

Is there a way to do it by creating an interface:

interface error {
     condition: boolean,
     message: string
}

using ngFor and iterating over a collection of errors, checking wether a condition is met?

The problem I have is that the condition is linked to function() : boolean, and ngFor does not renders itself again when the user enters an invalid input, since there is no change in the collection but in the value that the condition function will return.

Take a look at custom validators, angular.io/guide/form-validation#custom-validators. You can use these to set custom validation and then can listen for the error string and show your message. — Sam
– Sam, Commented Feb 17, 2020 at 13:27

Eliseo · Accepted Answer · 2020-02-17 13:40:36Z

2

when you create a customValidator you return an object if an error happens. This object can be different acording your requeriments.

  customError() {
    return (control: AbstractControl) => {
      if (isNaN(control.value)) return { error: "it's not a number" };
      if (+control.value > 100) return { error: "it's greater than 100" };
      return null;
    };
  }

   <div *ngIf="myform.get('control').errors">
         {{myform.get('control').errors.error}}
   </div>

You can also return an object with diferents property,e.g. errorNaN and errorGt

  customError() {
    return (control: AbstractControl) => {
      if (isNaN(control.value)) return { errorNaN : "it's not a number" };
      if (+control.value > 100) return { errorGt: "it's greater than 100" };
      return null;
    };
  }

   <div *ngIf="myform.get('control').errors?.errorNaN">
      It's not a number
   </div>
   <div *ngIf="myform.get('control').errors?.errorGt">
      it's greater than 100
   </div>

a simple example stackblitz

answered Feb 17, 2020 at 13:40

Eliseo

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3 Comments

Eliseo Over a year ago

Ben, the first example only show an unique error message and you has an unique div. The customError defined only give you an unique error

Ben Kalifa Over a year ago

But is there a way to loop over all of the existing errors, not manually creating div for each one but something like:. <div ngFor="let error ofmyform.get('control').errors">. <span>{{error.message}}</span>. </div>

Eliseo Over a year ago

the error is an object, so you need use the KeyValuePipe angular.io/api/common/KeyValuePipe, some like <div *ngFor="let error of myform.get('control').errors | keyvalue">{{error.key}}:{{error.value}}</div>

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