updating js object in lwc

Question

I have to change an attribute of a JSON object. Ex.

allOppList= [[{"A":"2051-11-21","B":"FLOAT"}]]

I have to change the B to "xyz" in the JS file.

I am doing by below code but I am not able to update the B element

let tempAllRecords = Object.assign([], this.allOppList);
        for (let j = 0; j < this.allOppList.length; j++) {
                let tempRec = Object.assign({}, tempAllRecords[j]);
                tempRec.B = event.detail.checked;
                tempAllRecords[j] = tempRec;
        }
        this.allOppList = tempAllRecords;

in this way, "B" of allOppList is not getting updated. Instead, it is getting updated like below

allOppList= [[{"A":"2051-11-21","B":"FLOAT"},"B":"New value"]]

Answer 1

As a performance matter, do not use for-index loops in LWC within Salesforce, or you'll suffer extreme performance penalties. You do have a nested loop, so the best way to do this is a nested map operation:

let tempAllRecords = this.allOppList.map(
  innerList => innerList.map(
    innerItem => ({ ...innerItem, B: event.detail.checked })
  )
);