Lookup an array against another with performance
You could create a look up table (a simple object) where you can pre calculate the amount of times a domain is in the repeated in the domains array, this reduces the algorithmic complexity of your current solution which is n * m (dataElements  * domainElements)
Whereas my suggestion has a lower complexity of (n + m)


const data = [
    {
        Domain: 'google.com',
        '# Reocurring Domains': 0
    },
    {
        Domain: 'apple.com',
        '# Reocurring Domains': 0
    },
    {
        Domain: 'facebook.com',
        '# Reocurring Domains': 0
    }
]

const domains = [
    {
        'google.com': true,
        'microsoft.com': true,
        'google.com': true
    },
    {
        'apple.com': true,
        'microsoft.com': true,
        'twitter.com': true
    },
    {
        'facebook.com': true,
        'apple.com': true,
        'facebook.com': true
    }
]

const domainsObj = {}

domains.forEach(domain =&gt; {
    Object.keys(domain).forEach(key =&gt; key in domainsObj ? domainsObj[key] += 1: domainsObj[key] = 1)
})

data.forEach(element =&gt; element['# Reocurring Domains'] = domainsObj[element.Domain]);

console.log(data);



This approach increases memory usage, but improves calculation speed.