odiff.js

function objectDiff(o1, o2) {
    o = {}
    for(let [key, value] of Object.entries(o2)) {
        if (value !== o1[key] && o1[key] !== undefined) {
            o[key] = value
        }
    }
    return o
}

function objectDiff2(o1, o2) {
    o = {}
    for(let key of Object.keys(o2)) {
        if (o2[key] !== o1[key] && o1[key] !== undefined) {
            o[key] = o2[key]
        }
    }
    return o
}

function objectDiff3(o1, o2) {
    o = {}
    for(let key of Object.keys(o2)) {
        const v1 = o1[key]
        if (o2[key] !== v1 && v1 !== undefined) {
            o[key] = o2[key]
        }
    }
    return o
}

a = {name: 'Johan', age: 33, len: 194, city: 'Lund'}
b = {age: 34, size: 8, city: 'Prag'}

// About 10ms
console.time('test')
console.log(objectDiff(a, b))
console.timeEnd('test')

// About 0.2 ms
console.time('test2')
console.log(objectDiff2(a, b))
console.timeEnd('test2')

// About 0.1-0.2 ms but always faster than test2
console.time('test3')
console.log(objectDiff3(a, b))
console.timeEnd('test3')

Like this:

$ node odiff.js
{ age: 34, city: 'Prag' }
test: 11.651ms
{ age: 34, city: 'Prag' }
test2: 0.245ms
{ age: 34, city: 'Prag' }
test3: 0.178ms