Two JS objects, get a value from first object that it missing in the second

Question

I have two JS objects:

var first = [{name: "Sam", group: "test"}, {name: "John", group: "it"}];
var second = [{name: "John", group: "it"}, {name: "Tim", group: "hr"}];
    
for (var k = 0; k < first.length; k++) {
    if (first.indexOf(second[k]) == -1) {
        console.log('found');
    }
}

I am trying to filter out a value that is present at first, but missing at second. How can I do that?

Ideal return value: {name: "Sam", group: "test"}

Tried many things, nothing works at the moment. For example:

Not clear what you're asking for. Are you trying to filter out based on the value of a particular property? For example, in your second array, none of the objects share any common values. — Geuis
– Geuis, Commented Jun 29, 2018 at 21:27

Ari Lotter · Accepted Answer · 2018-06-29 21:27:41Z

0

Using Array.filter, we can return items that pass a condition. In this case, the condition is "not in the second list", which we can express with Array.some.

function getItemsOnlyInFirst(first, second) {
  return first.filter(
    item =>
      !second.some(
        compareItem =>
          item.name === compareItem.name && item.group === compareItem.group
      )
  );
}

let first = [{ name: "Sam", group: "test" }, { name: "John", group: "it" }];
let second = [{ name: "John", group: "it" }, { name: "Tim", group: "hr" }];

getItemsOnlyInFirst(first, second); // {name: "Sam", group: "test"}

answered Jun 29, 2018 at 21:27

Ari Lotter

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Comments

Marco · Accepted Answer · 2018-06-29 21:27:58Z

The problem is you cannot compare two objects with the == operator:

var obj1 = {test: 1}
var obj2 = {test: 1}

console.log(obj1 == obj2) // false

An easy solution is to convert the objects into the JSON format.

This works because you can compare strings:

var obj1 = JSON.stringify({test: 1})
var obj2 = JSON.stringify({test: 1})

console.log(obj1 == obj2) // true

Solution using JSON.stringify:

var first  = [{name: "Sam", group: "test"}, {name: "John", group: "it"}];
var second = [{name: "John", group: "it"} , {name: "Tim", group: "hr"}];

// Convert the objects to JSON
var second_str = second.map(JSON.stringify)

var filtered = first.filter(function(elem) {
    return second_str.indexOf(JSON.stringify(elem)) == -1
})

console.log(filtered)

Maciej Kwas · Accepted Answer · 2018-06-29 21:45:25Z

0

I understand they both share same interface? For complex objects you could use lodash with eg: _.differenceBy function, but here the case is simple (still, approach is quite generic, does not depends on keys inside objects, but they need to be in flat structure [no recursion])

let isOnlyInFirst = first.filter(el => {
    return !second.some(z => {
        const el_keys = Object.keys(el)
        const z__keys = Object.keys(z)
        let flag = true
        for (const key of el_keys) {
            if (!z.hasOwnProperty(key) || z[key] != el[key]) flag = false
        }
        if (el_keys.length === z__keys.length && flag)
            return true
        return false
    })
})

answered Jun 29, 2018 at 21:45

Maciej Kwas

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