function injection

functionInjection.js

const transformData = dataArray =>{
  //transforms the data in some way
}


const dataToObj = dataArray =>{
  //converts the data to a new form
}

//The final object will be an object that contains all dependencies in this file.
//The functions get defaulted to the functions in the file, so it can easily be ommitted for writing non-test code
const composeData = (dataArray, {_dataToObj = dataToObj, _transformData = transformData} = {})=>{
  const transformedData = transformData(dataArray); 
  return dataToObj(transformedData); 
}


//What I like about this is that I get to worry about only compose data.
//I don't care what the other functions do. It's also really easy to test what happens under different circumstances
//Because I just state what I want the return value of each function to be. 
//This also doesn't disallow more heavy integration  or end to end tests. The main drawback is odd looking object at the end.
describe('composeData', ()=>{
  test('composeData should transform the data and then conver it to an obj', ()=>{
    const _transformData = jest.fn(()=> 'foo');
    const _dataToObj = jest.fn(()=> 'bar');
    const dataArray = [1,2,3];
    
    const result = composeData(dataArray, {_transformedData, _dataToObj}); 
    
    expect(_transformedData).toHaveBeenCalledWith([1,2,3]); 
    expect(_dataToObj).toHaveBeenCalledWith('foo'); 
    expect(result).toBe('bar'); 
  }
});