JSFiddle - React, Tailwind, and code Playground

JavaScript

var obj = {
    a: {
        b: ''
    }
};

function defined(ref, strNames) {
    var name;
    var arrNames = strNames.split('.');
    
    while (name = arrNames.shift()) {        
        if (!ref.hasOwnProperty(name)) return false;
        ref = ref[name];
    } 
    
    return true;
}

document.write( defined( obj, 'a.b' ) );