Why is my Global Variable Undefined Inside a JavaScript Function?

Surprised by Undefined Global Variables in JavaScript: Understanding Variable Hoisting

The behavior you encountered where a global variable is initially undefined within a function can be attributed to JavaScript's variable hoisting mechanism. JavaScript automatically lifts variable declarations to the top of their enclosing scope, even if the actual assignment of values occurs later in the code.

In the example provided, the global variable value is declared with an initial value of 10. However, within the test function, a local variable with the same name (value) is also declared without an assigned value (which defaults to undefined).

The first console.log statement in the function attempts to access the global value, but since the local value has already been hoisted, it takes precedence. As a result, the first call prints undefined. Subsequently, the second console.log statement within the function displays the value assigned to the local value variable (20).

This hoisting phenomenon applies to both variables and functions within their current execution context. It's essential to understand that only the declaration, not the assignment, is hoisted. This means that variables are accessible throughout their scope, but they may not have an assigned value until the code reaches the assignment statement.

For instance, consider the following code snippet:

var test = 'start';

function end() {
    test = 'end';
    var test = 'local';
}

end();
alert(test);

Surprisingly, this code would print start, despite the function end seemingly altering the value of the test variable. This is because within the end function, a new local variable declaration takes precedence, and the global variable remains unchanged.

While variable hoisting can simplify code, it's important to be aware of its implications to avoid unintended behavior. Properly scoping variables and fully understanding the hoisting mechanism can help maintain code structure and prevent potential errors.

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