reducing an array of nested objects in javascript

reduce method takes 2 important args: your reducing function and the default value at step 0 of the iteration

Here the reducer has 2 args:

• the so-called previousValue: here called reducerTarget as it receives what is returned in the body of the reducer
• the currentValue where the reducer is: here called currentArrayItem

Here's what you might want

    const {totalLCI, totalMCI} = data.reduce((reducerTarget, currentArrayItem) => { 
        return {
            totalLCI: reducerTarget.totalLCI + currentArrayItem.option.lci,
            totalMCI: reducerTarget.totalMCI + currentArrayItem.option.mci
        }
    }, {totalLCI: 0, totalMCI: 0})

From here, starting with the default value, what it does on the first iteration is

/* 1st step: 
    index = 0
    targetReducer = defaultValue = {totalMCI: 0, totalLCI: 0}
    currentArrayItem = {option: {lci: 1, mci: 2}}
*/
        return {
            totalLCI: 0 + 1,
            totalMCI: 0 + 2
        }

/* 2nd step: 
    index = 1
    targetReducer = previousValue = {totalMCI: 1, totalLCI: 2}
    currentArrayItem = {option: {lci: 3, mci: 4}}
*/
        return {
            totalLCI: 1 + 3,
            totalMCI: 2 + 4
        }

First you need know if you have stored value in the fields totalLCI and totalMCI, if they aren't set, add the previous value and the current value, after add, return the values, in the next iteration, you will check again if the data is stored in totalLCI and totalMCI, after that, you will get data stored in that field and sum it all the time till finish the iteration, here an example:

var data2 = [{ option:{ lci: 1, mci: 2 } }, { option:{ lci: 3, mci: 4 } }, { option:{ lci: 5, mci: 6 } }];

let test = data2.reduce( (previousValue, currentValue) => {
    
    if(!previousValue.totalLCI && !previousValue.totalMCI){

        return {
          totalLCI: previousValue.option.lci + currentValue.option.lci,
          totalMCI: previousValue.option.mci + currentValue.option.mci
        };
    }else{
        return{
            totalLCI: previousValue.totalLCI + currentValue.option.lci,
            totalMCI: previousValue.totalMCI + currentValue.option.mci
        }

    }
  });



console.log(test.totalLCI);
console.log(test.totalMCI);