Dynamic update (in-place) of a JSObject with dynamic path via array.

dynamic_object_updates.es6

const testOb = {
    slot1: {
        cubby1: {
            envelope1: {
                page1: "Hello World",
                page2: "Living large",
            },
            envelope2: {
                page1: "No Escape",
                page2: "Supercharger",
            },
        },
        cubby2: {
            envelope1: {
                page1: "Burger King",
                page2: "McDonalds",
            },
            envelope2: {
                page1: "Chili's",
                page2: "Olive Garden",
            },
        },
    },
    slot2: {
        cubby1: {
            envelope1: {
                page1: "Tomato",
                page2: "Potato",
            },
            envelope2: {
                page1: "Cherry",
                page2: "Grape",
            },
        },
        cubby2: {
            envelope1: {
                page1: "Parmesan",
                page2: "Swiss",
            },
            envelope2: {
                page1: "Chocolate",
                page2: "Vanilla",
            },
        },
    },
}

function updateValue(arrayPath, fieldName, value, overWrite) {
    let refOb = testOb;
    const fieldPath = [...arrayPath, fieldName];
    const len = fieldPath.length;
    for (let i=0; i<len-1; i++) {
        let elem = fieldPath[i];
        if( !refOb[elem] ) refOb[elem] = {}
        refOb = refOb[elem];
    }
    if (typeof(value) === 'number' && typeof(refOb[fieldPath[len-1]]) === 'number' && !overWrite) {
        refOb[fieldPath[len-1]] += value;
    } else {
        refOb[fieldPath[len-1]] = value;
    }
}

let itemPath = ["slot2","cubby1","envelope2"];
let fieldName = "page2";
// add an object
let newVal = ["Pineapple","ORANGE"];
console.log(`Before update : ${testOb.slot2.cubby1.envelope2.page2}`);
updateValue(itemPath, fieldName, newVal);
console.log(`After update1 : ${testOb.slot2.cubby1.envelope2.page2}`);
itemPath = [...itemPath];
// add a number
newVal = 10;
updateValue(itemPath, fieldName, newVal);
console.log(`After update2 : ${testOb.slot2.cubby1.envelope2.page2}`);
// increment the number
newVal = 21;
updateValue(itemPath, fieldName, newVal);
console.log(`After update3 : ${testOb.slot2.cubby1.envelope2.page2}`);
// decrement the number
newVal = -4;
updateValue(itemPath, fieldName, newVal);
console.log(`After update4 : ${testOb.slot2.cubby1.envelope2.page2}`);
// add a path that isn't there yet
itemPath = ["slot3","cubby2","envelope1"];
newVal = "Honda";
updateValue(itemPath, fieldName, newVal);
console.log(`After update5 : ${testOb?.slot3?.cubby2?.envelope1?.page2}`);
console.log(JSON.stringify(testOb));