Q:

Change ngForm field value

When i submit my form, i have this function 
login(form: NgForm)

If I try the following
console.log(form.value.password); // this works perfect.

but this doesn't work
form.value.password = ''

After failed login i want to reset the password field.

A:

but this doesn't work
form.value.password = ''

Because form.value returns you value of the form and it is an read only property part of AbstractControl
From angular2 source code
/**
 * The value of the control.
 */
readonly value: any;

You can use reset for your purpose. 
/**
   * Resets the control. Abstract method (implemented in sub-classes).
*/
abstract reset(value?: any, options?: Object): void;

Usage of setValue or patchValue. 
this.yourForm.controls['password'].setValue(password)

Or you can assign value to whole form 
this.yourForm.setValue(data);

When using setValue on whole form then you have ro provide every control otherwise it throw error like property missing paychValue in other hand can handle if you sending only on element.

