Why Are My Global JavaScript Variables Undefined Inside Functions?

JavaScript Variable Hoisting: Uncovering the Mystery of Undefined Global Variables

When working with JavaScript variables, it's easy to encounter surprising behavior. One such instance is when a global variable appears to have an undefined value within a certain function.

Example:

var value = 10;
function test() {
    console.log(value); // A
    var value = 20;
    console.log(value); // B
}
test();

Output:

undefined
20

Explanation:

The behavior stems from JavaScript Variable Hoisting, which automatically moves variable and function declarations to the top of the current scope. This means that:

• The global variable value is hoisted to the top of the test function.
• At point A, the program prints the value of the local value variable, which has not yet been initialized.
• At point B, the program correctly prints the value of the newly initialized local variable.

In effect, the code behaves as if it were written as:

var value;

function test() {
    console.log(value); // undefined
    value = 20;
    console.log(value); // 20
}

Side Note: Function declarations also undergo hoisting. This is why you can call a function before it is declared, unlike variable assignments.

Conclusion:

Variable hoisting should be considered when working with JavaScript variables. By understanding this behavior, developers can avoid unexpected undefined values in their code. Additionally, resources such as Ben Cherry's "JavaScript Scoping and Hoisting" can provide further insights into this fundamental aspect of JavaScript.

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