How to add a row to table conditionally?

Question

I am building an angular app where I will be adding an item to a table. The added row will be based on the input of an item name, and its quantity.

I am working on preventing the items being added into the table if:

• Nothing is selected or
• A non-positive number is entered into the quantity field (quantity field only accepts natural numbers)

An error message will show up if any of these conditions were not satisfied. I am able to get the error messages to show up, but a row with invalid fields were added.

Note: I am not allowed to disable the "Add Item" button if the conditions are not met.

Here is my code:

app.component.html

<form #order="ngForm">
    <table>
      <tr>
        <td>
          <button type="submit" class="btn btn-default" (click)="addItem()">Add
            Item</button>
        </td>
        <td>
          <select [(ngModel)]="newItem.name" required name="newItemName" #newItemName="ngModel" minlength="1">
            <option *ngFor="let o of options" [ngValue]="o">{{o.name}}</option>
          </select>
        </td>
        <td>Qty</td>
        <td>
          <input type="text" pattern="[0-9]*" required [(ngModel)]="newItem.quantity" 
          name="newItemQuantity" #newItemQuantity="ngModel">
        </td>
      </tr>
    </table>
    <p *ngIf="newItemName?.errors?.required && order.submitted">Please select an item.</p>
    <p *ngIf="newItemQuantity?.errors?.required && order.submitted">Quantity is required.</p>
  </form>
  <table>
    <thead>
      <tr>
        <th>Item</th>
        <th>Qty</th>
        <th>Unit Price</th>
        <th>Amount</th>
      </tr>
    </thead>
    <tbody>
      <tr *ngFor="let item of items; let i = index">
        <td><label class="form-control">{{item.name.name}}</label></td>
        <td><label class="form-control">{{item.quantity}}</label></td>
        <td><label class="form-control">{{item.name.price}}</label></td>
        <td></td>
        <td><input type="button" value="delete" (click)="removeItem(i)"></td>
      </tr>
    </tbody>
  </table>

app.component.ts

import { Component } from '@angular/core';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
})

export class AppComponent {
  items: Array<any> = [];
  newItem: any = {};
  options = [
    {name: "apples", price: 1.19},
    {name: "peaches", price: 1.39},
    {name: "pears", price: 1.69},
    {name: "plums", price: 1.59}
  ];

  addItem() {
    if (this.newItem != undefined) {
      this.items.push(this.newItem);
      this.newItem = {};
    }
  }

  removeItem(index) {
    this.items.splice(index, 1); // remove 1 item at ith place
  }

}

Answer 1

Just add a conditional at the begining of your add method:

addItem() {
  if(!this.newItem || !this.newItem.name || 
      (this.newItem && !this.newItem.name)) {
    return;
  }

  this.items.push(this.newItem);
  this.newItem = {};
}

Answer 2

You have to modify addItem() function to check if both name and quantity are defined and are valid. And only add the item if the condition is true.

addItem() {
  if (this.newItem && this.newItem.name && this.newItem.quantity &&
    this.newItem.quantity > 0 && Number.isInteger(Number(this.newItem.quantity))) {
    this.items.push(this.newItem);
    this.newItem = {};
  }
}

Number.isInteger() method can be used to check if quantity is a valid integer. This ensures that fractional numbers are not allowed.

Live demo on StackBlitz: https://stackblitz.com/edit/angular-vqcikf

Answer 3

Here is your answer, check wether the newItem object exists (not equal to undefined), then check for the name key & value exists (not equal to undefined and empty string ''), then check if quantity key & value exists (not equal to undefined and empty string ''), then make sure the quantity value is greater than 0.

addItem() {
  if(this.newItem && this.newItem.name && this.newItem.quantity && this.newItem.quantity > 0) {
    this.newItem.quantity = Number(this.newItem.quantity);
    this.items.push(this.newItem);
    this.newItem = {};
  }
}

Note: JS will take care of comparison between string data and numerical values

Example:

"1.11" > 1 //true
"0.01" < 0 //false
"1.0a" > 0 // undefined (which is evaluated as false)