Type enforcement in JavaScript

typeEnforcement.js

function User(data) {
    this.name = is(String)(data.name);
}

function is(obj) {
    return function(val) {
        if (val === null || val.constructor !== obj) {
            throw [
                'Invalid type specified; expected:',
                obj.name + ',',
                'got:',
                (val && val.constructor && val.constructor.name) || 'null'
            ].join(' ');
                
        }
        return val;
    }
}

var user = new User({name: 'James'});
console.log(user); // { name: 'James' }
console.log(user.constructor.name) // User

new User({name: null}); // Invalid type specified; expected: String, got: null