myHotTake

Tag: Angular form validation example

  • What’s the Best Way to Validate Multiple Angular Fields?

    If this story helps Angular make more sense, consider giving it a like or sharing it with someone diving into forms! 🌟

    Think of cross-field validation in Angular forms like managing a garden. Imagine I’m the gardener and my goal is to grow plants that thrive together, like sunflowers and tomatoes. Both plants are strong on their own, but together they share sunlight and nutrients differently. My job is to ensure they complement each other—if one takes too much water or shade, the other might suffer. That’s what cross-field validation feels like to me.

    In Angular, each form field is like a single plant in my garden. They have their own needs, like requiring water (a required field) or a certain type of soil (specific pattern). But sometimes, I need to look at the whole garden. For example, I can’t let the sunflowers block the sunlight from the tomatoes. That’s where a cross-field validator comes in—it’s the gardener’s rulebook for harmony.

    I create a custom validator that checks the entire form, like a guideline I set: “If the sunflower (password) grows too tall, make sure the tomatoes (confirm password) match its height.” This validator doesn’t just stop at one field; it looks across multiple plants and ensures balance.

    When I run my checks—calling this validator—it’s like taking a stroll through my garden, seeing if the rules are followed. If not, I leave a little marker (an error message) that says, “Hey, this plant needs adjusting!” Angular handles the hard work of making sure my feedback is visible to whoever’s looking after the garden next.

    That’s it. Cross-field validation feels like making sure my garden thrives as a whole, not just focusing on individual plants. 🌱

    Code Example

    First, we create a custom validator:

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

export const matchFieldsValidator: ValidatorFn = (control: AbstractControl): ValidationErrors | null => {
  const field1 = control.get('password'); // Sunflower
  const field2 = control.get('confirmPassword'); // Tomato

  if (!field1 || !field2) {
    return null; // If one field is missing, the check isn't valid
  }

  return field1.value === field2.value 
    ? null 
    : { fieldsMismatch: true }; // Error if they don't match
};

    Here, control.get('password') and control.get('confirmPassword') represent the two plants. The rule ensures the sunflower (password) and tomato (confirm password) are at the same height (their values match).

    Next, we apply this validator to the form group:

    import { FormGroup, FormBuilder } from '@angular/forms';

constructor(private fb: FormBuilder) {}

form = this.fb.group(
  {
    password: [''],
    confirmPassword: [''],
  },
  { validators: matchFieldsValidator } // Apply the validator here
);

    The validator is tied to the entire form group, meaning it looks at the “garden” as a whole.

    Finally, in the template, we show an error message if the fields don’t match:

    <div *ngIf="form.hasError('fieldsMismatch')">
  Passwords must match!
</div>

    How It Works

    • Field 1 and Field 2: These are your individual plants.
    • Custom Validator: This is your gardener’s rulebook, checking the whole form (garden).
    • Error Handling: Angular helps display the gardener’s “marker” when something goes off.

    Key Takeaways

    1. Cross-field validation is holistic: It checks the form group, not individual fields.
    2. Use a custom validator for reusable logic: You can write it once and apply it across forms.
    3. Angular handles error visibility: Once the validator flags an issue, you can show tailored messages.

    Final thought: Writing cross-field validators isn’t just about enforcing rules—it’s about creating harmony in your data. Keep nurturing that garden of yours, and it’ll thrive! 🌱

  • How Do You Create Custom Validators in Angular?

    If you enjoy this explanation or find it helpful, give it a like or share it—it helps me know what resonates with you!

    I’m a guardian of a magical bridge that only allows worthy travelers to cross. The bridge has some basic rules: you must be human, over 18 years old, and carrying a glowing gem. These rules are the “default validators”—Angular already knows how to enforce them.

    But then, one day, a wise elder tells me, “Guardian, the glowing gem rule is too vague. Some gems may glow, but they’re fake! You need a new test—only real enchanted gems should be allowed.”

    I think, “Okay, I’ll make my own test!” I create a magnifying glass that I call the ‘isEnchantedGem’ test. This test looks at a gem and checks for specific sparkles only enchanted gems have. Once I build the magnifying glass, I attach it to the bridge’s gatekeeper system.

    In Angular, I’d do the same thing for a form. Forms have default validators like “required” or “minimum length,” but if I need a custom rule—like checking for enchanted gems—I write a custom validator function. This function is the magnifying glass: it looks at the input and decides if it passes my unique test.

    Here’s the code version of my enchanted gem test:

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

export function isEnchantedGem(control: AbstractControl): ValidationErrors | null {
  const value = control.value;
  const isValid = value && value.includes('sparkles');
  return isValid ? null : { notEnchanted: true };
}

    I give this to the gatekeeper—my form control—so every gem (user input) is inspected for enchantment.

    And just like that, I’ve tailored the bridge rules for exactly what I need. Custom validators in Angular are your magnifying glass for input. Now, whether it’s enchanted gems or any other unique check, your form can handle it with style!

    Sure, let’s dive into Part 2 and connect the story back to JavaScript and Angular with some code examples.

    Part 2: Building the Enchanted Bridge Validator in Angular

    So, remember my enchanted bridge and its magical validator? Let’s see how we bring that story to life with JavaScript in Angular.

    Here’s the deal: in Angular, custom validators are just functions that follow a specific structure. They inspect the control (input field) and return an object with error details if the validation fails. If everything’s fine, they return null.

    Creating the Custom Validator

    Let’s write a validator function for our “enchanted gem” check. The input field should contain the word sparkles to be valid:

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

// Our "isEnchantedGem" validator function
export function isEnchantedGem(): ValidatorFn {
  return (control: AbstractControl): ValidationErrors | null => {
    const value = control.value;
    const isValid = value && value.includes('sparkles');
    return isValid ? null : { notEnchanted: true }; // Error if invalid
  };
}

    Here’s what’s happening:

    1. ValidatorFn: This is the type Angular uses for custom validators. It ensures our function returns either ValidationErrors or null.
    2. Error Object: If the input is invalid, we return { notEnchanted: true }. This key-value pair helps identify the specific error.
    3. Control: The control parameter represents the form input being validated.

    Using the Validator

    Now let’s attach this validator to a form field. Angular’s Reactive Forms make this simple:

    import { FormBuilder, FormGroup, Validators } from '@angular/forms';
import { isEnchantedGem } from './custom-validators';

export class EnchantedBridgeComponent {
  magicalForm: FormGroup;

  constructor(private fb: FormBuilder) {
    this.magicalForm = this.fb.group({
      gem: ['', [Validators.required, isEnchantedGem()]], // Attach custom validator here
    });
  }

  onSubmit() {
    if (this.magicalForm.valid) {
      console.log('Welcome, traveler! Your gem is enchanted.');
    } else {
      console.log('Sorry, traveler. Your gem is not valid.');
    }
  }
}

    Here:

    • We attach isEnchantedGem() to the gem field.
    • If the field fails validation, Angular flags it, and we can display an error to the user.

    Displaying Validation Errors

    To show users why their input failed, we use Angular’s template tools:

    <form [formGroup]="magicalForm" (ngSubmit)="onSubmit()">
  <label for="gem">Gem:</label>
  <input id="gem" formControlName="gem" />
  <div *ngIf="magicalForm.get('gem')?.errors?.notEnchanted">
    This gem is not enchanted! Add some sparkles.
  </div>
  <button type="submit">Cross the Bridge</button>
</form>

    Here, the error object { notEnchanted: true } becomes the key we check in the template.

    Key Takeaways:

    1. Custom Validators: They’re just functions that validate input and return ValidationErrors | null.
    2. Reusable Logic: Encapsulate specific checks like isEnchantedGem into validators for clean, reusable code.
    3. Error Feedback: Use Angular’s reactive forms and error objects to dynamically show users why their input is invalid.
    4. Flexibility: Custom validators allow you to tailor validation logic to unique requirements beyond the defaults like required or minLength.