When a variable is assigned an undefined or null value, it has no properties. Trying to access properties of such a variable anyway results in a TypeError, causing abrupt termination of the script if the error is not caught in a catch block. But instead of catch-ing this condition, it is best to avoid it altogether.

Noncompliant Code Example

if (x === undefined) {
  console.log(x.length); // Noncompliant; TypeError will be thrown
}

See