Writing Scripts in Nashorn - Scripting in Java: Integrating with Groovy and JavaScript - page 131

Consider the code and its output in Listing 4-21. The output will surprise you if you
have not worked with JavaScript before.

Listing 4-21. Declaration Hoisting in Nashorn

// beforehoisting.js

// Try accessing empId variable before declaring it
print("1. empId = " + empId);

// Declare and initialize empId now
var empId = 100;

function test() {
if (empId === undefined) {
var empId = 200;
}
print("2. empId = " + empId);
}

// Call the test function
test();

print("3. empId = " + empId);

1. empId = undefined
2. empId = 200
3. empId = 100

The first output shows that you can access a variable before declaring it. The variable
value is undefined . Inside the test() function, you would expect that expression empId
=== undefined will read the value of empId from the global scope and it will evaluate
to false . However, the output shows otherwise. The second output shows that the if
condition inside the test() function evaluated to true and the empId local variable was
set to 200. The third output shows that the function call did not affect the global variable
empId and its value remained 100.
You can understand this variable's scoping behavior in two ways:

• As a two-phase process : First, all declarations are processed.

Second, the code is executed. Because all declarations are
processed before executing the code, all variable and function
declarations are available when the code is executed, irrespective
of the place where they were declared. Of course, you need to take
into account the global and function scopes of those declarations.