Angular 2 FormGroup Add Validators dynamic

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Try this, it should work

this.formGroup.controls["firstName"].setValidators(Validators.required);

For multiple validators

this.formGroup.controls["firstName"].setValidators([Validators.required, Validators.minLength(2)]);

But doing so will override any validators that are provided during initialization

EDIT :

To reflect the form controls with newly added Validators immediately, we have to call this.formGroup.controls["firstName"].updateValueAndValidity(); after setting validators dynamically.

this.formGroup.controls["firstName"].setValidators(Validators.required);
this.formGroup.controls["firstName"].updateValueAndValidity();

DEMO for the same

* NOTE *

updateValueAndValidity() will trigger a valueChanges event even if the value didn’t really change (potentially resulting in an infinite loop, if you’re calling it from within a valueChanges subscription). You can prevent that by using the optional parameter object: { onlySelf: true, emitEvent: false}

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