How to check if user email/name is taken

REST API
8

Can someone explain why this script doesn’t work in user.register.post.pre_process?

var lodash = require("lodash.min.js");

if (event.request.payload.resource) {  // use 'payload' for request

lodash._.each(event.request.payload.resource, function( record ) {

    if (record.first_name === '') {
        throw 'Name field is required';
    }
});
}

If I register a user with first_name = “” it succeeds.