Comparing Two JavaScript Objects and Pulling out Key and Value of Matching Property

Question

If I know there's a property in common between two JavaScript objects called "req" and "updatedDoc" respectively, is there a way I can use a placeholder parameter to represent any key, so that I can find the right one that matches on the two objects? I tried this but it doesn't work:

for (const [key, val] of Object.entries(req)) {
  if (key === updatedDoc[key]) {
    console.log("key, val", key, val);
  }
}

By the way, in my use case I know there will always be one matching property between the two objects. And to clarify, the two objects are called "req" and "updatedDoc". I don't know what they key will be, but I know the two objects will have one key in common.

To add a little more clarity, "req" is going to be something simple, like:

const req = { 
  "deleted" : true, 
  "apiKey" : "4d9d9291", 
  "token" : "ffdsfjsdfsdjfa" 
}

... whereas updatedDoc will be a full document, like this:

const updatedDoc = {
   _id: <ObjectId>,
   firstName: "John",
   lastName: "Smith",
   age: 42
   deleted: false
}

Both have a property called "deleted". Basically I'm matching a request passed in with the whole document it pertains to. I then want to take in the value from "req" and save it to "updatedDoc" for the correct key. But first I need to find the matching key, and pull out the value from "req". Is there a way I can do this?

Are you only testing for a property AND value that matches? or just the keys. If just the keys, any of these would work: stackoverflow.com/questions/12433604/… to get you the matches, which you can then use to get the values etc. — user400654
– user400654, Commented Feb 22, 2019 at 20:50
yeah, so just grab the keys of each and do a diff. If you know the one object will only have one key, it becomes even simpler; no need to loop. — user400654
– user400654, Commented Feb 22, 2019 at 20:53
key === updatedDoc[key] I think the problem here is that by this way you compare the key from req with the value from updatedDoc , while you want to compare the key of these two objects — Osama
– Osama, Commented Feb 22, 2019 at 20:53

Khauri · Accepted Answer · 2019-02-22 21:32:39Z

2

You should be able to just modify your loop to change

if (key === updatedDoc[key]) to if (key in updatedDoc)

Everything inside this if statement will only be performed on keys that exist both in req and updatedDoc. The value stored for the key in req will be val, which was dereferenced from Object.entries

You can change updatedData to the new value like so updatedData[key] = val. You can also store the change in an array for later, if you like.

const updatedDoc = {
  firstName: "John",
  lastName: "Smith",
  age: 42,
  deleted: false
}
const req = {
  "deleted": true,
  "apiKey": "4d9d9291",
  "token": "ffdsfjsdfsdjfa"
}

const changes = []

for (const [key, val] of Object.entries(req)) {
  if (key in updatedDoc) {
    // get the previous value
    const oldVal = updatedDoc[key]
    // update updatedDoc
    updatedDoc[key] = val
    // store the change or do whatever
    changes.push({
      [key]: {
        new: val,
        old: oldVal
      }
    })
  }
}

console.log(updatedDoc)

console.log(changes)

answered Feb 22, 2019 at 21:32

Khauri

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1 Comment

Muirik Over a year ago

I like this, too. Nice one.

Nina Scholz · Accepted Answer · 2019-02-22 21:22:24Z

1

Why not take a Set for the first object and filter the keys for the second object.

The result is an array with common keys.

var objectA = { a: 1, b: 2, c: 3 },
    objectB = { b: 4, c: 5, d: 6 },
    common = Object.keys(objectB).filter(Set.prototype.has, new Set(Object.keys(objectA))),
    values = common.map(k => objectA[k]);

console.log(common);
console.log(values);

A bit shorter version.

var objectA = { a: 1, b: 2, c: 3 },
    objectB = { b: 4, c: 5, d: 6 },
    common = Object.keys(objectA).filter({}.hasOwnProperty.bind(objectB)),
    values = common.map(k => objectA[k]);

console.log(common);
console.log(values);

edited Feb 22, 2019 at 21:22

answered Feb 22, 2019 at 21:01

Nina Scholz

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3 Comments

Muirik Over a year ago

So this gives me the keys in common, which is good, but I'd also need to know the value for that matching key found in req. So, in your case here: [ { "b" : 4 }, { "c" : 5} ], assuming objectB represents req.

Nina Scholz Over a year ago

you could map the values, please see edit. do you want an object with common key and values form objectA?

Muirik Over a year ago

Nice JavaScript Jiu Jitsu here. I think this will work, thanks.

Muirik · Accepted Answer · 2019-02-22 21:15:58Z

0

Try this solution I think it will solve the problem

updatedDoc.forEach(function(i,v){
for (const [key, val] of Object.entries(req)) {
      if (key === i && req[key]==updatedDoc[i]) {
            console.log("key, val", key, val);
      }
});

edited Feb 22, 2019 at 21:15

Muirik

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answered Feb 22, 2019 at 21:03

Osama

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