How to get name of input field from Angular2 FormControl object?

To expand on Radim Köhler answer. here is a shorter way of writing that function.

getControlName(c: AbstractControl): string | null {    const formGroup = c.parent.controls;    return Object.keys(formGroup).find(name => c === formGroup[name]) || null;}

We can use .parent property, well today ["_parent"] (see more below):

export const getControlName = (control: ng.forms.AbstractControl) =>{    var controlName = null;    var parent = control["_parent"];    // only such parent, which is FormGroup, has a dictionary     // with control-names as a key and a form-control as a value    if (parent instanceof ng.forms.FormGroup)    {        // now we will iterate those keys (i.e. names of controls)        Object.keys(parent.controls).forEach((name) =>        {            // and compare the passed control and             // a child control of a parent - with provided name (we iterate them all)            if (control === parent.controls[name])            {                // both are same: control passed to Validator                //  and this child - are the same references                controlName = name;            }        });    }    // we either found a name or simply return null    return controlName;}

and now we are ready to adjust our validator definition

public asyncValidator(control: FormControl): {[key: string]: any} {  //var theFieldName = control.someMethodOfGettingTheName(); // this is the missing piece  var theFieldName = getControlName(control);   ...

.parent later, ["_parent"] now

At the moment (today, now), current release is :

2.1.2 (2016-10-27)

But following this issue: feat(forms): make 'parent' a public property of 'AbstractControl'

And as already stated here

2.2.0-beta.0 (2016-10-20)

Features

• forms: make 'parent' a public property of 'AbstractControl' (#11855) (445e592)
• ...

we could later change the ["_parent"] into .parent

As of Angular 4.2.x, you can access a FormControl's parent FormGroup (and its controls) using the public parent property:

private formControl: FormControl;//...Object.keys(this.formControl.parent.controls).forEach((key: string) => {  // ...});