Angular 12 - TypeError: Cannot read properties of null (reading 'writeValue')
I'm creating a generic text input component, everything works well while serving the project, but in the built project I get this error:
TypeError: Cannot read properties of null (reading 'writeValue')
HTML:
<div class="p-field">
<label>{{label}} <span class="p-error">{{checkRequired(ngControl.control) ? '*' : ''}}</span></label>
<input class="full-width" [type]="type" pInputText [formControl]="ngControl.control">
<small *ngIf="ngControl.control.touched && ngControl.control.errors?.required" class="p-error">{{label}} is required</small>
<small *ngIf="ngControl.control.errors?.minlength" class="p-error">
{{label}} must be at least {{ngControl.control.errors.minlength['requiredLength']}}
</small>
<small *ngIf="ngControl.control.errors?.maxlength" class="p-error">
{{label}} must be at most {{ngControl.control.errors.maxlength['requiredLength']}}
</small>
<small *ngIf="ngControl.control.errors?.email" class="p-error">
This email is not valid
</small>
<small *ngIf="ngControl.control.errors?.isMatching" class="p-error">Passwords do not match</small>
</div>
component.ts
import { Component, Input, OnInit, Self } from '@angular/core';
import {AbstractControl, ControlValueAccessor, NgControl} from '@angular/forms';
@Component({
selector: 'app-text-input',
templateUrl: './text-input.component.html',
styleUrls: ['./text-input.component.css']
})
export class TextInputComponent implements ControlValueAccessor {
@Input() label: string;
@Input() type = 'text';
constructor(@Self() public ngControl: NgControl) {
this.ngControl.valueAccessor = this;
}
checkRequired(control) {
if (control.validator) {
const validator = control.validator({} as AbstractControl);
if (validator && validator.required) {
return true;
}
}
}
writeValue(obj: any): void {
}
registerOnChange(fn: any): void {
}
registerOnTouched(fn: any): void {
}
}
Angular info:
Angular CLI: 12.2.6 Node: 14.17.3 Package Manager: npm 6.14.13 OS: win32 x64 Angular: 12.2.6
The error occurs when provider setup is missing
providers: [
{
provide: NG_VALUE_ACCESSOR,
multi:true,
useExisting: CUSTOMER_INPUT_CLASS_HERE
}
]
In your case
@Component({
selector: 'app-text-input',
templateUrl: './text-input.component.html',
styleUrls: ['./text-input.component.css'],
providers: [
{
provide: NG_VALUE_ACCESSOR,
multi:true,
useExisting: TextInputComponent
}
]
})
export class TextInputComponent implements ControlValueAccessor {
...
}
I got this error when i tried to make the authorsiation check in the backend part of the validation logic. I declared the submit button in the formbuilder group. When I used an input with style="display: none" all was fine.
In constructor use @Optional() and do null check :
constructor(@Optional() @Self() public ngControl: NgControl) {
if (this.ngControl != null) this.ngControl.valueAccessor = this;
}
Also, add body to methods like
writeValue(value: number): void {
this.value = value;
this.onChange(this.value);
}
onChange: (_: any) => void = (_: any) => {};
onTouched: () => void = () => {};
registerOnChange(fn: any): void {
this.onChange = fn;
}
registerOnTouched(fn: any): void {
this.onTouched = fn;
}