Angular 2 Pass parameter to a validator function

Just move it to a class

class NpiNumValicator {
  constructor(private someField: someType) {}

  npiNumValidator(control: Control): { [s: string]: boolean } {
    if (isNaN(control.value)) {
        return { npiNAN: true };
    }
  }
}

then use it like

this.clientForm = _fb.group({ 'name': ['',
    Validators.compose([Validators.required])], 'npi': ['', 
    Validators.compose([Validators.required, 
        new NpiNumValidator(someArg).npiNumValidator, 
        Validators.maxLength(10), Validators.minLength(10)
    ])
]});

to be able to use this within npiNumValidator you can use

var npiNumVal = new NpiNumValidator(someArg);
this.clientForm = _fb.group({ 'name': ['',
    Validators.compose([Validators.required])], 'npi': ['', 
    Validators.compose([Validators.required, 
        npiNumVal.npiNumValidator.bind(npiNumVal), 
        Validators.maxLength(10), Validators.minLength(10)
    ])
]}); 

You also leverage a method of your component to create the validation function. This way you will be able to access properties of this component using an arrow function.

Here is a sample:

@Component({ ... })
export class MyComponent {
  constructor(private fb:FormBuilder) {
    this.form = this.fb.group({
      fieldName: [ '', this.createValidator() ]
    });
  }

  createValidator() {
    return (control) =>  {
      var arr = this.arr;
      (...)
    };
  }
}

Is it possible to add parameters to the function calls inside of the "compose" function?

Validator Declaration: straight out of Angular Code

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

static minLength(minLength: number): ValidatorFn {
    return (control: modelModule.AbstractControl): {[key: string]: any} => {
      if (isPresent(Validators.required(control))) return null;
      var v: string = control.value;
      return v.length < minLength ?
                 {"minlength": {"requiredLength": minLength, "actualLength": v.length}} :
                 null;
    };
  }

Usage:

Validators.compose([Validators.minLength(4)]

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NOTE: to understand it better, see How do JavaScript closures work?