I can't figure out why the option value is being displayed as "2: Object" when printing it. Can someone please clarify

Hey there! Currently, I have a UI built using Angular 5 and TypeScript. In one of my components, I have a reactive form with a select box form control in the view. What I'm trying to achieve is that whenever a different option is selected from the select box, it should display the chosen value. For this purpose, I have implemented a function called contactListChanged in my component class.

<select formControlName="contactList" [compareWith]="compareResource" (change)="contactListChanged($event)">
<option value="" disabled>{{ 'PLACEHOLDERS.CONTACT_LIST' | translate }}</option>
<option *ngFor="let contactList of contactLists" [ngValue]="contactList">{{ contactList.name }}</option>
</select>

In the component's TypeScript file:

contactListChanged($event){
    let selectedValue  = $event.target.value ;
    console.log(selectedValue);// prints 1: Object

    console.log(this.contactListControl.value); // prints Object {id: "1", name: "irish msisdn 1", total: 1000, displayName: "irish msisdn 2(1000)"}
}

The line console.log(selectedValue) displays the value as "2: Object". However, console.log(this.contactListControl.value) shows the correct value as Object {id: "2", name: "irish msisdn 2", total: 1200, displayName: "irish msisdn 2(1200)"}

Why does this happen? I expected console.log(selectedValue) to display the correct value, similar to console.log(this.contactListControl.value).

My goal is to use [ngValue] and retrieve the selected object in the contactListChanged method. How can I accomplish this?

Any help would be greatly appreciated!

Thank you.

Answer №1

By default, Javascript does not natively support storing objects or arrays in the value attribute of the select element. However, Angular has cleverly addressed this limitation through the use of ngValue. When utilizing ngValue with objects in Angular, the value of the select element in Javascript is set to id: Object, where id represents the index of the selected option and Object is a plain string value. Therefore, if you try to access the value of the selected option using pure Javascript ($event.target.value), you will receive outputs like 1: Object, 2: Object, etc., depending on the selected option. Angular takes care of mapping the object for you.

Instead of relying on plain Javascript, it is recommended to access the selected value through Angular's built-in binding methods such as formControl or ngModel. Angular seamlessly extracts the index (id) from the Javascript value (id: Object) and matches it to the options in the select dropdown to retrieve the object value.

Therefore, for proper value retrieval, it is best to utilize formControl or ngModel when accessing the selected value. As you are working with reactive forms, your implementation already showcases the correct way to retrieve the value.

contactListChanged() {
  const selectedValue = this.contactListControl.value;
  console.log(selectedValue);
}

get contactListControl() {
  return this.form.get('contactList');
}

(Source: Angular codebase)

Answer №2

When binding the contactList to [ngValue]="contactList", the selected object in contactListChanged($event) is determined by connecting the name property of contactList to ngValue as shown below:

<option *ngFor="let contactList of contactLists" [ngValue]="contactList.name">{{ contactList.name }}</option>

If you only need to pass a string, you can use value instead. The key difference is that value is always a string, while ngValue allows you to pass an object.

Answer №3

It's crucial to recognize that values obtained from input elements (such as radio buttons, checkboxes, and select options) are always converted to type string.

Since contactList is an object, the console is displaying it as such: an object converted to a string.

If you want the value to represent the specific object you desire, use the object's id instead like this: [value]="contactList.id" (or any other property that uniquely identifies each object):

<option *ngFor="let contactList of contactLists" [value]="contactList.id">{{ contactList.name }}</option>

Then, utilize the id to retrieve the desired object:

contactListChanged($event) {
    let objId = $event.target.value;
    console.log(this.contactLists.find(x => x.id == objId));
}

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