JavaScript - Optimizing a two array.map loop
That'd be with map:

this.countries = this.countries.map(country =&gt; {
    var concession = concessions.find(x =&gt; x.address.country === country.iso);
    country.address = concession.address;
    country.slug = concession.slug;
    return country;
});


That'd be with forEach:

this.countries.forEach(country =&gt; {
    var concession = concessions.find(x =&gt; x.address.country === country.iso);
    country.address = concession.address;
    country.slug = concession.slug;
});


I don't know if you can be sure there always be a concession for each country and how you should proceed in case there isn't.