how to write a validator in angular code example

Example 1: custom validator

import {AbstractControl, ValidatorFn} from '@angular/forms';

export function blue(): ValidatorFn {  
    return (control: AbstractControl): { [key: string]: any } | null =>  
        control.value?.toLowerCase() === 'blue' 
            ? null : {wrongColor: control.value};
}
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Example 2: formgroup check if valid

form: FormGroup;

onSubmit(){
    //checks if form is valid
       if( this.form.valid){
          //more code here
        }
}