ValidatorFn minLength(num minLength)

Validator that requires controls to have a value of a minimum length.

Source 

static ValidatorFn minLength(num minLength) {
  return /* Map < String , dynamic > */ (model_module
      .AbstractControl control) {
    if (Validators.required(control) != null) return null;
    String v = control.value;
    return v.length < minLength
        ? {
            "minlength": {
              "requiredLength": minLength,
              "actualLength": v.length
            }
          }
        : null;
  };
}