Uncaught ReferenceError: x is not defined

ReferenceError: x is not defined means the code reads a name that no reachable scope declares. JavaScript looks the identifier up from the current block outwards to the global scope; when nothing called x exists, it throws. Declare the variable (let/const) in a scope that contains the use, fix a typo, or load the script that defines the global before your code runs. In browsers the console shows it as Uncaught ReferenceError: x is not defined; in Node.js the process exits with code 1.

Reproduce it

cat > app.js <<'EOF'
function sum(values) {
  let total = 0;
  for (const v of values) total += v;
  return total;
}
sum([1, 2, 3]);
console.log(total);   // total only exists inside sum()
EOF
node app.js 2>&1 | grep '^ReferenceError'
exit ${PIPESTATUS[0]}

ReferenceError: total is not defined

total is declared with let inside sum(), so it does not exist at the top level.

Fix: use the returned value or widen the scope

cat > app.js <<'EOF'
function sum(values) {
  let total = 0;
  for (const v of values) total += v;
  return total;
}
const total = sum([1, 2, 3]);
console.log(total);

// Optional globals: typeof does not throw for undeclared names
console.log(typeof jQuery === 'undefined' ? 'jQuery not loaded' : 'jQuery loaded');
EOF
node app.js

6
jQuery not loaded

Common variants