Calculating difference between 2 items in JavaScript object

I have some customer data in an array of javascript objects that looks as such:

  const clientData = [
    {
      client: 'Pizza Hutt',
      food_cost: 400.00,
      prev_year_food_cost: 450.00,
    },
    {
      client: 'Pizza World',
      food_cost: 500.00,
      prev_year_food_cost: 650.00,
    },
  ];

I would like to loop over and get the difference between food_cost and prev_year_food_cost for each client. So ideally, I would have a new object containing client, food_cost and the difference.

[
    {
      client: 'Pizza Hutt',
      food_cost: 400.00,
      difference: 50.00,
    },
    {
      client: 'Pizza World',
      food_cost: 500.00,
      difference: 150.00,
    },
];

I have tried this but its not quite right. It’s pseudo code.

const comparisonData = [];
    for (let i = 0; i < data.length; i += 1) {
      const item = { ...data[i] };
      // console.log(item);
      Object.keys(item).forEach((key) => {
        if (key !== 'campaignname' && !key.includes(comparisonPeriodData.excluded_period)) {
             Do calculations here
          }
        }
      });
      push data onto comparisonData
    }

Answer

You can use Array#map to apply the transformation over each object, using Math.abs to find the absolute difference.

We can use destructuring to remove the prev_year_food_cost from the object.

This can be made more configurable by using the bracket syntax:

let clientData = [{ client: 'Pizza Hutt', food_cost: 400.00, prev_year_food_cost: 450.00, }, { client: 'Pizza World', food_cost: 500.00, prev_year_food_cost: 650.00, }];

const difference = (arr, key) => {
  return arr.map(o => {
    const { ["prev_year_" + key]: prev, ...rest} = o;
    return { ...rest,
       difference: Math.abs(o["prev_year_" + key] - o[key])
    }
  });
}
console.log(difference(clientData, "food_cost"));

clientData = [{ client: 'Pizza Hutt', water_cost: 400.00, prev_year_water_cost: 450.00, }, { client: 'Pizza World', water_cost: 500.00, prev_year_water_cost: 650.00, }];

console.log(difference(clientData, "water_cost"));