Hoisting and Execution Context

Variables assigned without using var, let, or const are implicitly declared as global variables (if not in strict mode). When you assign a value to a variable that hasn't been formally declared, JavaScript automatically creates that variable in the global scope, regardless of where the assignment happens. For example: ```javascript function test() { x = 10; // x is not declared with var, let, or const console.log(x); // 10 } test(); console.log(x); // 10 - x is available globally ``` This behavior can lead to unexpected bugs and is generally considered a bad practice, as it can accidentally overwrite existing global variables and makes code harder to maintain. That's why 'strict mode' was introduced - when you enable strict mode by adding `'use strict';` at the top of your script or function, assigning to undeclared variables will throw a ReferenceError instead: ```javascript 'use strict'; function test() { x = 10; // ReferenceError: x is not defined } ``` It's always recommended to explicitly declare variables using var, let, or const to clearly indicate their intended scope and avoid potential issues.