CODEWITHSUNDEEP
javascript
Lucas Martini
Lucas Martini

Reputation: 183

dynamically set object value with undefined key

I am trying to create an object that has no value on it.

for (let i = 0; i < chartyearsale.length; i++) {

    var year = chartyearsale[i].year,
        grp = chartyearsale[i].family,
        qnt = chartyearsale[i].qnt,
        qntsk = chartyearsale[i].qntsk,
        fat = chartyearsale[i].total;

    total[year] = Object.assign({
        [grp]: {
            val1: (total[year][grp].val1 || 0) + val1,
            val2: (total[year][grp].val2 || 0) + val2,
            val3: (total[year][grp].val3 || 0) + val3
        }
    }, total[year]);

}

The values "year, group, value1, value2 and value3" are all defined.

I am getting this response:

Cannot read properties of undefined (reading 'grp')

I believe this should be done differently:

(total[year][grp].val1 || 0)
//should return 0 if undefined, but it breaks the script!

Upvotes: 0

Views: 812

Answers (1)

CertainPerformance
CertainPerformance

Reputation: 370679

You can't access a nested property that doesn't exist, even if you alternate it with || 0 on the right-hand side:

const obj = {};

// Forbidden:
console.log(obj.foo.bar);

So doing total[year][group].val1 fails, because total starts out as the empty object. You need

val1: (total[year]?.[group]?.val1 ?? 0) + value1,

for all three values, to make the nested access safe.

A nicer approach would be, if you're creating the object for the first time:

const total = {
    [year]: {
        [group]: {
            val1: value1,
            val2: value2,
            val3: value3,
        }
    }
};

If the properties may already exist:

total[year] ??= {};
total[year][group] ??= {};
const totalGroup = total[year][group];
totalGroup.val1 = (totalGroup.val1 ?? 0) + value1;
totalGroup.val2 = (totalGroup.val2 ?? 0) + value2;
totalGroup.val3 = (totalGroup.val3 ?? 0) + value3;

Upvotes: 2

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