FormControl returning empty value in Angular 2

I am using a form

<form [formGroup]="form">
<input type="text" value={{userDetails.first_name}} formControlName = "firstName">
<button type="submit" data-action="save" (click)="onSubmit(form.value)">
</form>


Here userDetails.first_name value is Tom. It is visible on UI inside text box ; but when I submit form form.firstName is giving me empty value.

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