Angular - Using the 'name' attribute with the 'mat-select' element

Question

Angular - Using the 'name' attribute with the 'mat-select' element

Currently, I am working on an Angular form that involves the dynamic nature of the userEntitiesRoles array. To ensure smooth functionality, each mat-select tag within the ngFor loop requires a unique name attribute. In order to achieve this, I attempted to generate random strings using the TypeScript code provided below. However, upon testing, I encountered an error in Chrome's console as outlined at the end of this post. Any insights or suggestions on how to resolve this issue?

html:

<div *ngFor="let item of userEntitiesRoles">
    <mat-form-field>
      <mat-label>{{'USERS.ROLE' | translate}}</mat-label>
      <mat-select name="role-{{generateRandomString(5)}}" [(ngModel)]="item.role" required>
        <mat-option [value]="optionRole" *ngFor="let optionRole of roleOptions">{{optionRole | uppercase}}</mat-option>
      </mat-select>
    </mat-form-field>
</div>

Typescript:

generateRandomString(length) {
    var result           = '';
    var characters       = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    var charactersLength = characters.length;
    for ( var i = 0; i < length; i++ ) {
       result += characters.charAt(Math.floor(Math.random() * charactersLength));
    }
    return result;
 }

error:

ERROR Error: NG0100: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value: 'zAx5R'. Current value: 'rPFo6'..

angular typescript

Answer 1

Answer №1

This particular area is quite vast and may require several hours of research to fully grasp. In essence, Angular performs two checks on the DOM during initialization; first to assign values and the second time to validate that the initial assignment was correct.

During both checks, the function generateRandomString(5) will be invoked, yielding a different result each time it runs. Angular interprets this as indicating a potential issue in your code, hence the error message you receive.

While this issue might not occur in production mode, it is advisable to address it regardless.

One potential solution:

It is generally recommended not to embed functions directly in your HTML, as they are triggered every time an event occurs in the DOM (such as mouse movements, clicks, or scrolls). Instead, consider using variables or arrays that can be initialized in ngOnInit or whenever they need updating, and then modify your HTML accordingly:

<div *ngFor="let item of userEntitiesRoles; index as i">
    <mat-form-field>
      <mat-label>{{'USERS.ROLE' | translate}}</mat-label>
      <mat-select name="role-{{myRandomArray[i]}}" [(ngModel)]="item.role" required>
        <mat-option [value]="optionRole" *ngFor="let optionRole of roleOptions">{{optionRole | uppercase}}</mat-option>
      </mat-select>
    </mat-form-field>

Another suggested approach:

Utilize ChangeDetectorRef and invoke it in AfterViewInit

constructor(private cd: ChangeDetectorRef) {
}

ngAfterViewInit() {
    this.cd.detectChanges();
}

The second validation occurs before ngAfterViewInit is executed, and invoking cd.detectChanges() prompts Angular to reevaluate. This method works effectively with variables that change between validations but may not yield expected results with functions.

Combining both approaches may offer a comprehensive resolution to this matter.

Note that while I am not an expert, some nuances in my explanation may exist. For further details, refer to this informative article on the topic AngularInDepth

Answer 2

This particular area is quite vast and may require several hours of research to fully grasp. In essence, Angular performs two checks on the DOM during initialization; first to assign values and the second time to validate that the initial assignment was correct.

During both checks, the function generateRandomString(5) will be invoked, yielding a different result each time it runs. Angular interprets this as indicating a potential issue in your code, hence the error message you receive.

While this issue might not occur in production mode, it is advisable to address it regardless.

One potential solution:

It is generally recommended not to embed functions directly in your HTML, as they are triggered every time an event occurs in the DOM (such as mouse movements, clicks, or scrolls). Instead, consider using variables or arrays that can be initialized in ngOnInit or whenever they need updating, and then modify your HTML accordingly:

<div *ngFor="let item of userEntitiesRoles; index as i">
    <mat-form-field>
      <mat-label>{{'USERS.ROLE' | translate}}</mat-label>
      <mat-select name="role-{{myRandomArray[i]}}" [(ngModel)]="item.role" required>
        <mat-option [value]="optionRole" *ngFor="let optionRole of roleOptions">{{optionRole | uppercase}}</mat-option>
      </mat-select>
    </mat-form-field>

Another suggested approach:

Utilize ChangeDetectorRef and invoke it in AfterViewInit

constructor(private cd: ChangeDetectorRef) {
}

ngAfterViewInit() {
    this.cd.detectChanges();
}

The second validation occurs before ngAfterViewInit is executed, and invoking cd.detectChanges() prompts Angular to reevaluate. This method works effectively with variables that change between validations but may not yield expected results with functions.

Combining both approaches may offer a comprehensive resolution to this matter.

Note that while I am not an expert, some nuances in my explanation may exist. For further details, refer to this informative article on the topic AngularInDepth

Answer 3

Answer №2

When the error you described occurs, it means that changes are being made after Angular has finished checking for any updates. The life cycle hooks have already run and then a new change is detected. To fix this problem, you can manually initiate change detection by injecting ChangeDetectorRef (let's call it cdr) in the constructor and then using this.cdr.detectChanges(). This function helps to manually start the change detection cycle again.

Answer 4

When the error you described occurs, it means that changes are being made after Angular has finished checking for any updates. The life cycle hooks have already run and then a new change is detected. To fix this problem, you can manually initiate change detection by injecting ChangeDetectorRef (let's call it cdr) in the constructor and then using this.cdr.detectChanges(). This function helps to manually start the change detection cycle again.

Angular - Using the 'name' attribute with the 'mat-select' element

Answer №1

Answer №2

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