JSFiddle - React, Tailwind, and code Playground

JavaScript

foo = { jon : {age: 'old',   feeling: 'sad' },
        jack: {age: 'young', feeling: 'happy'}
       };
bar = { name: 'jon' };

function isMember(bar) {
    for(prop in foo) {
        console.log([prop, foo[prop], bar.name, ]);
        if (prop === bar.name){
            return true;
        }
    }
    return false;
};

console.log(['ismember', isMember(bar)]);