How to "migrate" one object to another object in JavaScript?

Question

Given two object, named: ObjectA and ObjectB :

var ObjectA = {
    config1: "5",
    config2: "6",
    config3: "7"
}
var ObjectB = {
    config1: "1",
    config2: "2",
    config4: "3",
    config5: "4"
}

How can I copy from ObjectB to ObjectA so that ObjectA have the same properties of ObjectB while the values of properties in ObjectA is kept and values from ObjectB that doesn't exist in ObjectA is copied to ObjectA ?

Expected result:

ObjectA = {
    config1: "5",
    config2: "6",
    config4: "3",
    config5: "4"
}

• config3 is deleted since it doesn't exist in ObjectB ,
• config4 with config5 are copied to ObjectA since those doesn't exist in ObjectA ,
• and value in config1 with config2 are kept the same in ObjectA since those two exist in both of the object)

Answer 1

Use reduce :

ObjectA = Object.keys(ObjectB).reduce(
    (acc, key) => ({ ...acc, [key]: ObjectA[key] ?? ObjectB[key]}), {}
)

This will use the keys from ObjectB , take values from ObjectA , when they exist (with fallback to ObjectB ), and populate an empty object with the results.

Answer 2

You can use the reducer function to solve.

 var ObjectA = { config1: "5", config2: "6", config3: "7" } var ObjectB = { config1: "1", config2: "2", config4: "3", config5: "4" } const res = Object.keys(ObjectB).reduce((prev, next) => { if(ObjectA[next]){ prev[next] = ObjectA[next]; } return prev; }, {...ObjectB}) console.log(res)

Answer 3

Here's a possible solution that is more old school: What this basically do:

• Loop over the properties in ObjectA. If it doesn't exist in ObjectB, delete
• Loop over the properties in ObjectB. If it doesn't exist in ObjectA, copy it.

function mergeObject(oldObj: Object, newObj: Object) {
    var copyOfOldObj = Object.assign({}, oldObj);
    for (const key in copyOfOldObj) {
        if (!newObj.hasOwnProperty(key)) {
            delete copyOfOldObj[key];
        }
    }
    for (const key in newObj) {
        if (!copyOfOldObj.hasOwnProperty(key)) {
            copyOfOldObj[key] = newObj[key];
        }
    }
    return copyOfOldObj;
}