js impl of bind

bind2.js

// tslint:disable no-any

export default function bind2(this: any, that: any) {
  const f = this // tslint:disable-line no-this-assignment
  const initialArgs = Array.prototype.slice.call(arguments, 1)
  
  return (finalArgs: any[]) => f.apply(that, [...initialArgs, ...finalArgs])
}

tests|bind2.test.js

// tslint:disable no-any
import bind2 from '../bind2'

function decorate(obj: any): any {
  const res = new Function()
  Object.assign(res, obj)
  res['bind2'] = bind2 // tslint:disable-line no-string-literal
  return res
}

describe('.bind2(target)', () => {
  test('works', () => {
    const data = {
      getX() {
        return this.x
      },
      x: 42,
    }
    const unboundGetX = data.getX
    const boundGetX = decorate(unboundGetX).bind2(data)

    expect(boundGetX()).toEqual(42)
  })
})

     Expected value to equal:
          42
     Received:
          undefined