Get the difference two objects by subtracting properties

Question

I am trying to get the difference between two objects

previousChart: {BWP: 1, ZAR: 1.3, USD: 0.09324, number: 1},
currentChart: {BWP: 1, ZAR: 1.35, USD: 0.01, number: 2}

The desired answer is:

newObject ={BWP: 0, ZAR: 0.05, USD: 0.08324, number: -1}

Please don't ask me what I have done as this is the last stage because this is the last part of my code, if you are interested in knowing what I have done here it is:

rates = [
{BWP: 1, ZAR: 1.3, USD: 0.09324, number: 1},
{BWP: 1, ZAR: 1.35, USD: 0.01, number: 2},
{BWP: 1, ZAR: 1.3456, USD: 0.09234, number: 3},
{BWP: 1, ZAR: 1.27894, USD: 0.06788, number: 4}
]

newRate = [];

for(let i in rates){

    if( i - 1 === -1 ){
        previousChart = rates[0];
    }else{
        previousChart = rates[i - 1];
    }
    let currentChart = rates[i];

}

Answer 1

You can just loop through the Object.keys() of one object and subtract the other using reduce(). This assumes both objects have the same keys.

let previousChart =  {BWP: 1, ZAR: 1.3, USD: 0.09324, number: 1};
let currentChart = {BWP: 1, ZAR: 1.35, USD: 0.01, number: 2};

let newObj = Object.keys(previousChart).reduce((a, k) => {
    a[k] = previousChart[k] - currentChart[k];
    return a;
}, {});

console.log(newObj);

Of course you can add some code to handle the floating points to the precision you want.

You can make this into a function, which will allow you to easily work with an array of values and subtract them all:

let rates = [
    {BWP: 1, ZAR: 1.3, USD: 0.09324, number: 1},
    {BWP: 1, ZAR: 1.35, USD: 0.01, number: 2},
    {BWP: 1, ZAR: 1.3456, USD: 0.09234, number: 3},
    {BWP: 1, ZAR: 1.27894, USD: 0.06788, number: 4}
];

function subtract(r1, r2) {
  return Object.keys(r1).reduce((a, k) => {
      a[k] = r1[k] - r2[k];
      return a;
  }, {});
}

let total = rates.reduce((a, c) => subtract(a, c));

// subtract all values of rates array
console.log(total);