How to calculate value from multiple inputs using angular reactive forms?

Question

I'm trying to calculate either discount percent or discount amount from list price using angular 6 reactive forms .

form.component.html (simplified)

...
  <form [formGroup]="productForm">
    <mat-form-field class="quat-width">
      <span matPrefix>$ &nbsp;</span>
      <input matInput type="number" name="CostPrice" formControlName="CostPrice">
    </mat-form-field>
    <mat-form-field class="quat-width">
      <span matPrefix>$ &nbsp;</span>
      <input matInput type="number" name="ListPrice" formControlName="ListPrice">
    </mat-form-field>
    <mat-form-field class="quat-width">
      <span matPrefix>$ &nbsp;</span>
      <input matInput type="number" name="DiscountAmount" formControlName="DiscountAmount">
    </mat-form-field>
    <mat-form-field class="quat-width">
      <span matPrefix>% &nbsp;</span>
      <input matInput type="number" name="DiscountPercent" formControlName="DiscountPercent">
    </mat-form-field>
  </form>

form.component.ts (simplified)

...
  export class ProductFormComponent implements OnInit {

    productForm: FormGroup;

    constructor(
      private fb: FormBuilder,
      private productService: ProductsService) { }

    ngOnInit() {
      this.createForm();
    }

    createForm() {
      this.productForm = this.fb.group({
        DiscountAmount: [0],
        DiscountPercent: [0],
        DiscountPrice: [0],
        ListPrice: [0],
      });
      this.productForm.valueChanges.debounce(250).subscribe(val =>{
        if (val.ListPrice > 0 && val.DiscountAmount > 0) {
          const newVal = Math.round((val.ListPrice / val.DiscountAmount) * 100);
          this.productForm.controls.DiscountPercent.patchValue(newVal);
        }
        if (val.ListPrice > 0 && val.DiscountAmount > 0) {
          const newVal = Math.round((discPercent / 100) * val.ListPrice);
          this.productForm.controls.DiscountAmount.patchValue(newVal);
        }
      }  
      this.validateForm();
    }

    validateForm() {
      Object.keys(this.productForm.controls).forEach(key => {
        this.productForm.controls[key].markAsTouched();
        this.productForm.controls[key].updateValueAndValidity();
      });
    }
  }

the code above is what I got so far and doesn't work, creates a maximum callstack reached error.

How can I set the value of discount amount if list price and discount percent is changed, and set the value of the discount percent when list price and discount amount is changed?

Answer 1

you can use {emitEvent: false} option for patchValue and updateValueAndValidity

 this.productForm.valueChanges.subscribe(val =>{
        if (val.ListPrice > 0 && val.DiscountAmount > 0) {
          const newVal = Math.round((val.ListPrice / val.DiscountAmount) * 100);
          this.productForm.controls.DiscountPercent.patchValue(newVal, {emitEvent: false});
        }
        if (val.ListPrice > 0 && val.DiscountAmount > 0) {
          const newVal = Math.round((val.DiscountPercent / 100) * val.ListPrice);
          this.productForm.controls.DiscountAmount.patchValue(newVal, {emitEvent: false});
        }
       this.validateForm();   
      });


    }

    validateForm() {
      Object.keys(this.productForm.controls).forEach(key => {
        this.productForm.controls[key].markAsTouched();
        this.productForm.controls[key].updateValueAndValidity({emitEvent: false});
      });
    }