Example Codes Demonstrating the "Cannot assign to a reference or variable!" Error in Angular 4

This error arises in Angular 4 applications when you attempt to modify a value that's designated as a reference or a template variable within your component's template. These variables are intended to hold references to DOM elements or component instances, and their values shouldn't be directly changed.

Common Causes:

Resolving the Error:

Example (Incorrect):

<input type="text" #myInput [(ngModel)]="myValue" (change)="myValue = 'modified'">

<input type="text" #myInput [(ngModel)]="myValue" (change)="onInputChange()">

// In your component class
import { Component } from '@angular/core';

@Component({
  // ...
})
export class MyComponent {
  myValue = '';

  onInputChange() {
    // Modify myValue here (e.g., this.myValue += ' (modified)')
  }
}

Example Codes Demonstrating the "Cannot assign to a reference or variable!" Error in Angular 4

This code snippet attempts to change the value of a template reference variable (#myInput) directly within the template, leading to the error:

<input type="text" #myInput value="Initial value" (change)="myInput.value = 'Modified value'">

Explanation:

Here, #myInput creates a reference variable for the input element. However, assigning "Modified value" to myInput.value inside the (change) event handler is incorrect. Template reference variables are designed to provide a fixed reference to the element, not to be modified directly.

Correct Code (Event Binding and Component Method):

This code demonstrates the proper way to handle changes based on user interaction:

<input type="text" #myInput value="Initial value" (change)="onInputChange()">

// In your component class
import { Component } from '@angular/core';

@Component({
  // ...
})
export class MyComponent {
  myValue = 'Initial value'; // Component property for two-way data binding

  onInputChange() {
    this.myValue = 'Modified value'; // Modify the component property
  }
}

• The template uses (change) to trigger the onInputChange() method in the component class when the input value changes.
• The onInputChange() method updates the myValue property in the component, which is bound using two-way data binding ([(ngModel)]="myValue").
• This ensures that changes are made to the underlying component data, and the input element automatically reflects the updated value.

Incorrect Code (Modifying Bound Property with Two-Way Data Binding):

This code attempts to modify the bound property (myValue) directly within the template, causing the error:

<input type="text" [(ngModel)]="myValue" (change)="myValue = 'something else'">

The [(ngModel)] directive establishes two-way data binding between the myValue property in your component and the input element. Assigning a new value to myValue within the template disrupts this synchronization.

Correct Code (Two-Way Data Binding):

In this corrected code, modifications are made to the component property (myValue):

<input type="text" [(ngModel)]="myValue">

// In your component class
import { Component } from '@angular/core';

@Component({
  // ...
})
export class MyComponent {
  myValue = '';

  // Handle other events or logic within component methods as needed
}

The template simply uses [(ngModel)] for two-way data binding. Changes to the input element's value will automatically update the myValue property, and vice versa.

1. Using Local Variables with Event Bindings:

   • If you need temporary data within the template for calculations or manipulations based on user input, you can introduce a local variable with the let keyword and modify it within the template.

   <input type="text" (keyup)="calculateSomething(let value = $event.target.value)">
   

   • In this example, value is a local variable that holds the current input value. You can use it for calculations within the template or pass it to a component method via event binding.

2. Leveraging ngModelChange with Input Property:

   • For advanced use cases involving custom input controls or complex interactions, you might consider using the (ngModelChange) event along with an input property in your component.

   <custom-input [(ngModel)]="myValue" (ngModelChange)="handleCustomInputChange($event)"></custom-input>
   

   • custom-input is your custom component.
   • (ngModelChange) emits the new value whenever the input changes.
   • The handleCustomInputChange method in your component receives the new value and can handle any necessary logic.

   Note: This approach requires more complex component interaction and might not be suitable for simple scenarios.

3. Template Expressions and Conditional Rendering:

   • In some cases, you might use template expressions and conditional rendering to manipulate the appearance of elements based on the value of a reference or bound property. However, this technique should be used cautiously to avoid cluttering the template with logic.

   <span *ngIf="myValue === 'modified'">Value has been modified!</span>
   

   • Here, the <span> element is conditionally rendered only when myValue is equal to "modified".