"use strict";

    let undef;
    let str1 = "hello";
    let str2 = str1 + undef;
    console.log(str2); // -> "helloundefined"

    "use sane";

    let undef;
    let str1 = "hello";
    let str2 = str1 + undef; // -> Uncaught TypeError: Cannot append string and undefined
    console.log(str2); // (code never reaches this point)