DistinctBy operator written in Typescript

I am extended my Linq library for Typescript with many more methods! Here is my implementation of DistinctBy.

if (!Array.prototype.DistinctBy) {
  Array.prototype.DistinctBy = function <T>(property: (keyof T)): T[] {
    if (this === null || this === undefined) {
      return [];
    }
    let filteringArray = this.Select(property).map(n => n[property]);

    let distinctRunOnArray = this.filter((value, index, array) => {
      let valueProperty = value[property];
      return filteringArray.indexOf(valueProperty) === index;
    });
    return distinctRunOnArray;
  }
}

This Jasmine test can test this operator out.

describe('TSLinq Jasmine tests', () => {

  it('can filter out duplicates using DistinctBy on array of items of objects', () => {
    let someArray: Student[] = [];
    someArray.push(<Student>{ StudentID: 1, StudentName: "John", Age: 13 });
    someArray.push(<Student>{ StudentID: 2, StudentName: "Moin", Age: 21 });
    someArray.push(<Student>{ StudentID: 2, StudentName: "Moin", Age: 21 });
    someArray.push(<Student>{ StudentID: 4, StudentName: "Ram", Age: 20 });
    someArray.push(<Student>{ StudentID: 5, StudentName: "Ron", Age: 15 });
    let expectedArray: Student[] = [];
    expectedArray.push(<Student>{ StudentID: 1, StudentName: "John", Age: 13 });
    expectedArray.push(<Student>{ StudentID: 2, StudentName: "Moin", Age: 21 });
    expectedArray.push(<Student>{ StudentID: 4, StudentName: "Ram", Age: 20 });
    expectedArray.push(<Student>{ StudentID: 5, StudentName: "Ron", Age: 15 });
    let result = someArray.DistinctBy<Student>("StudentID");
    expect(result).toEqual(expectedArray);
  });


});

The Student class is simple:

class Student {
  StudentID: number;
  StudentName: string;
  Age: number;
}

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