JSFiddle - React, Tailwind, and code Playground

JavaScript

var response = {
    "message":"The request is invalid.",
    "modelState":{
        "model.Email":["The Email must be at least 10 characters long."],
        "model.ConfirmPassword":["The password and confirmation password do not match."]
    }
}

var priorities = ["model.Email", "model.ConfirmPassword"];
var selectedError = null,
    idx = 0;

do {
    selectedError = response.modelState[ priorities[ idx ] ];
    selectedError = selectedError ? selectedError[0] : null;
    idx++;
} while ( !selectedError && idx < priorities.length );

console.log( selectedError || 'Unknown error'  );