What is the result of using an undeclared variable in non-strict mode?

Using an undeclared variable in non-strict mode leads to the creation of a global variable. This behavior is a key distinction of non-strict mode in JavaScript. When a variable is assigned a value without being declared with keywords like var, let, or const, JavaScript interprets this as an intention to create a property on the global object, which is typically the window object in a browser environment.

For instance, if you write x = 10; in a non-strict mode context, JavaScript automatically creates a global variable named x and assigns it the value 10. This can lead to unexpected behavior and potential conflicts, especially in larger applications, as it allows for variables to be accessible from anywhere in the code without being explicitly declared.

In contrast, in strict mode, using an undeclared variable would raise a ReferenceError, thus enforcing better coding practices and avoiding accidental global variable creation. Additionally, the variable would not remain undefined or trigger a TypeError; instead, it would simply not exist until declared properly. This distinction highlights why understanding the behavior of variable declaration in different modes is crucial for JavaScript developers.