Angular 6 custom validation return type

Question

I have this code in my project for custom validation but I am unable to understand what does: (group: FormGroup) => ValidationResult mean here?

I understand: (group: FormGroup) means return type but then what does => ValidationResult mean here?

static comparePasswords(passwordKey: string, confirmPasswordKey: string): (group: FormGroup) => ValidationResult {
    return (group: FormGroup): ValidationResult => {
      const password = group.controls[passwordKey];
      const confirmPassword = group.controls[confirmPasswordKey];
      if (password.value !== confirmPassword.value) {
        return { mismatchedPasswords: true };
      }
    };
  }

Answer 1

The full return type is (group: FormGroup) => ValidationResult . That is, the return type is a function that takes a FormGroup argument and returns a ValidationResult .

This is true since you are indeed returning a function that takes a FormGroup and returns a ValidationResult . The return type of the internal function is ValidationResult .

Answer 2

I understand : (group: FormGroup) means return type but then what does => ValidationResult mean here?

No. The return type is (group: FormGroup) => ValidationResult

Which means that the comparePasswords method returns a function that takes a FormGroup as argument and returns a ValidationResult . Ie it's a FormGroup validator.

Answer 3

You can see this code like the following :

{functionName}( {params}[] ) : {returnType} {
    {functionBody}
}

Where your functionName is comparePasswords
Your params is passwordKey: string, confirmPasswordKey: string
your returnType is (group: FormGroup) => ValidationResult

The type returned by your function is also a function.

When you execute comparePasswords , you get a function back.

If you execute this function, passing it a FormGroup , you will get a ValidationResult

Answer 4

In this block, it is returning a function which accepts an argument of type FormGroup and returns ValidationResult

Usually, in custom validation, we usually return null if the validation succeeds and return a key value pair denoted by type {[key:string]:string}

This key is used in the errors collection which you can use to validate whether your validation failed or succeeded