reducing an array of nested objects in javascript - nested

Question

so i have this array and i want to return the sum of lci and the sum of mci with a reduce function
const data = [{ option:{ lci: 1, mci: 2 } }, { option:{ lci: 3, mci: 4 } }, { option:{ lci: 5, mci: 6 } }];
i have tried this but it's not working:
data.reduce( (previousValue, currentValue) => {
return {
totalLCI: previousValue.option.lci + currentValue.option.lci,
totalMCI: previousValue.option.lci + currentValue.option.lci,
};
})

Answer 1

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
}

Answer 2

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);