Disable (make read-only) text input on mat-datepicker when using a reactive form

To create a disabled FormControl is really simple.

1 - Don't use disabled attribute in your template;

2 - Instantiate your FormGroup like this:

this.formGroup = this.formBuilder.group({
  dateJoined: { disabled: true, value: '' }
  // ...
});

---

That being said, although you want to prevent users from typing something in the input, you still want to let them select a date by clicking the button (more specifically in matSuffix), right?

If it's correct, disable doesn't work for this case, because it'll disable all the input (including the button in matSuffix).

To solve your case, you can use readonly. Instantiate the FormGroup normally and then in template:

<input                          
  matInput 
  readonly <- HERE
  [matDatepicker]="dateJoined" 
  placeholder="Date joined" 
  formControlName="dateJoined">

DEMO

To expand on developer033's answer, to dynamically toggle the readonly state of the input, modify the readonly property using a component variable.

Dynamically Toggle Readonly

See Stackblitz Demo

app.component.html

<input matInput 
  [readonly]="inputReadonly"   <!-- Update `readonly` property using variable -->
  [matDatepicker]="dateJoined" 
  placeholder="Date joined" 
  formControlName="dateJoined">

app.component.ts

import {Component} from '@angular/core';
import {FormBuilder, FormGroup, Validators} from '@angular/forms';
import {VERSION} from '@angular/material';

@Component({
  selector: 'material-app',
  templateUrl: 'app.component.html'
})
export class AppComponent {
  formGroup: FormGroup;
  inputReadonly = true;
  version = VERSION;

  constructor(private formBuilder: FormBuilder) { }

  ngOnInit() {
    this.formGroup = this.formBuilder.group({
      dateJoined: ''
    });
  }

  public toggleInputReadonly() {
    this.inputReadonly = !this.inputReadonly;
  }
}