Assigning to an undeclared variable

Model answer. In non-strict ("sloppy") code, assigning to an undeclared name creates a property on the global object (window in browsers, globalThis everywhere). It becomes an accidental global that any script can read or overwrite, which causes hard-to-trace bugs:

function addTax(price) {
  total = price * 1.18;   // no declaration: leaks a global called total
  return total;
}

In strict mode ('use strict', and automatically inside ES modules and classes), the same line throws ReferenceError: total is not defined. That turns a silent bug into an immediate error.

Note that reading an undeclared name throws in both modes; only writing differs. typeof undeclaredName is the one safe way to check, because it returns "undefined" instead of throwing.

Fix: always declare variables, and write modules (which are strict by default).