Deep Property Access with Typescript

deep-property-access.spec.ts

import {DeepPropertyAccess} from './deep-property-access';

interface Customer {
  name: string;
  company?: {
    name: string;
    address?: {
      city: string;
    }
  };
}

describe('DeepPropertyAccess', () => {
  const completeCustomer: Customer = {
    name: 'Stefan',
    company: {
      name: 'Company',
      address: {
        city: 'Vienna'
      }
    }
  };
  const nameOnlyCustomer: Customer = {
    name: 'Stefan'
  };

  it('should get the city "Vienna"', () => {
    const city = DeepPropertyAccess.get(completeCustomer, 'company', 'address', 'city');
    expect(city).toBe('Vienna');
  });

  it('should return undefined because city is not defined', () => {
    const city = DeepPropertyAccess.get(nameOnlyCustomer, 'company', 'address', 'city');
    expect(city).toBe(undefined);
  });

  it('should return undefined because city is not defined', () => {
    const city = DeepPropertyAccess.get(nameOnlyCustomer, 'company', 'address', 'city');
    expect(city).toBe(undefined);
  });

  it('should have the correct type for city', () => {
    const city = DeepPropertyAccess.get(completeCustomer, 'company', 'address', 'city');
    expect(typeof city).toBe('string');
  });

  it('should not detect "number" as type of city', () => {
    const city = DeepPropertyAccess.get(completeCustomer, 'company', 'address', 'city');
    expect(typeof city).not.toBe('number');
  });

  it('should extract the correct value 1 levels deep', () => {
    const level_2 = {
      level_1: ['Hello']
    };
    const result = DeepPropertyAccess.get(level_2, 'level_1');
    expect(result.length).toBe(1);
    expect(result[0]).toBe(<any> 'Hello');
  });

  it('should extract the correct value 2 levels deep', () => {
    const level_3 = {
      level_2: {
        level_1: ['Hello']
      }
    };
    const result = DeepPropertyAccess.get(level_3, 'level_2', 'level_1');
    expect(result.length).toBe(1);
    expect(result[0]).toBe(<any> 'Hello');
  });

  it('should extract the correct value 3 levels deep', () => {
    const level_4 = {
      level_3: {
        level_2: {
          level_1: ['Hello']
        }
      }
    };
    const result = DeepPropertyAccess.get(level_4, 'level_3', 'level_2', 'level_1');
    expect(result.length).toBe(1);
    expect(result[0]).toBe(<any> 'Hello');
  });

  it('should extract the correct value 4 levels deep', () => {
    const level_5 = {
      level_4: {
        level_3: {
          level_2: {
            level_1: ['Hello']
          }
        }
      }
    };
    const result = DeepPropertyAccess.get(level_5.level_4, 'level_3', 'level_2', 'level_1');
    expect(result.length).toBe(1);
    expect(result[0]).toBe(<any> 'Hello');
  });

  it('should extract the correct value 5 levels deep', () => {
    const level_6 = {
      level_5: {
        level_4: {
          level_3: {
            level_2: {
              level_1: ['Hello']
            }
          }
        }
      }
    };

    // TODO: found out why it has to be cast, when it is 5 levels deep
    const result = <Array<any>> <any> DeepPropertyAccess.get(level_6, 'level_5', 'level_4', 'level_3', 'level_2', 'level_1');
    expect(result.length).toBe(1);
    expect(result[0]).toBe(<any> 'Hello');
  });
});

deep-property-access.ts

// Original code: https://codewithstyle.info/Deep-property-access-in-TypeScript/
// thanks to Miłosz (https://disqus.com/by/miloszpiechocki/ ) for fixing it
export class DeepPropertyAccess {

  public static get<T,
    P1 extends keyof NonNullable<T>>(obj: T, prop1: P1): NonNullable<T>[P1] | undefined;

  // tslint:disable:max-line-length
  public static get<T,
    P1 extends keyof NonNullable<T>,
    P2 extends keyof NonNullable<NonNullable<T>[P1]>>(obj: T, prop1: P1, prop2: P2): NonNullable<NonNullable<T>[P1]>[P2] | undefined;

  public static get<T,
    P1 extends keyof NonNullable<T>,
    P2 extends keyof NonNullable<NonNullable<T>[P1]>,
    P3 extends keyof NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>>(obj: T, prop1: P1, prop2: P2, prop3: P3): NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3] | undefined;

  public static get<T,
    P1 extends keyof NonNullable<T>,
    P2 extends keyof NonNullable<NonNullable<T>[P1]>,
    P3 extends keyof NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>,
    P4 extends keyof NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>>(obj: T, prop1: P1, prop2: P2, prop3: P3, prop4: P4): NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>[P4] | undefined;

  public static get<T,
    P1 extends keyof NonNullable<T>,
    P2 extends keyof NonNullable<NonNullable<T>[P1]>,
    P3 extends keyof NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>,
    P4 extends keyof NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>,
    P5 extends keyof NonNullable<NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>[P4]>>(obj: T, prop1: P1, prop2: P2, prop3: P3, prop4: P4, prop5: P5): NonNullable<NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>[P4]>[P5] | undefined;

  public static get<T,
    P1 extends keyof NonNullable<T>,
    P2 extends keyof NonNullable<NonNullable<T>[P1]>,
    P3 extends keyof NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>,
    P4 extends keyof NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>,
    P5 extends keyof NonNullable<NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>[P4]>,
    P6 extends keyof NonNullable<NonNullable<NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>[P4]>[P5]>>(obj: T, prop1: P1, prop2: P2, prop3: P3, prop4: P4, prop5: P5, prop6: P6): NonNullable<NonNullable<NonNullable<NonNullable<NonNullable<NonNullable<T>[P1]>[P2]>[P3]>[P4]>[P5]>[P6] | undefined;
  /* tslint:enable:max-line-length */

  // ...and so on...

  // the actual function to extract the property
  public static get(obj: any, ...props: string[]): any {
    return obj && props.reduce(
      (result, prop) => result == null ? undefined : result[prop],
      obj
    );
  }
}