What is the output of the function getAge when 'use strict' is applied?

In JavaScript, applying 'use strict' enforces stricter parsing and error handling on your JavaScript code. One of the significant effects of 'use strict' is that it prevents the use of undeclared variables. If a variable is created without the var, let, or const keyword, it becomes a global variable in non-strict mode but will throw a ReferenceError in strict mode.

If the function getAge references a variable that was not declared within its scope (and hence would be undefined in a non-strict context), in a strict context, JavaScript will raise a ReferenceError when it encounters this undeclared variable. This prevents silent errors and helps developers catch mistakes in their code early by enforcing better coding practices regarding variable declarations.

Thus, if getAge is attempting to access a variable that wasn’t declared, the output will result in a ReferenceError under 'use strict', as strict mode does not allow the use of undeclared variables. This understanding is critical for writing robust JavaScript code compliant with modern standards and practices.