javascript

Chapter 4: Variables, Scope, and Memory
Here, the function add() defines a local variable named sum that contains the result of an addition
operation. This value is returned as the function value, but the variable sum isn ’ t accessible outside the
function. If the var keyword is omitted from this example, sum becomes accessible after add() has been
called, as shown here:
function add(num1, num2) {
sum = num1 + num2;
return sum;
}
var result = add(10, 20); //30
alert(sum); //30
Here, the variable sum is initialized to a value without ever having been declared using var . When
add() is called, sum is created in the global context and continues to exist even after the function has
completed, allowing you to access it later.
Initializing variables without declaring them is a very common mistake in
JavaScript programming and can lead to errors. It ’ s advisable to always declare
variables before initializing them to avoid such issues.
Identifier Lookup
When an identifier is referenced for either reading or writing within a particular context, a search must
take place to determine what identifier it represents. The search starts at the front of the scope chain,
looking for an identifier with the given name. If it finds that identifier name in the local context, then
the search stops and the variable is set; if the search doesn ’ t find the variable name, it continues along
the scope chain. This process continues until the search reaches the global context ’ s variable object. If the
identifier isn ’ t found there, it hasn ’ t been declared.
To better illustrate how identifier lookup occurs, consider the following example:
var color = “blue”;
function getColor(){
return color;
}
alert(getColor()); //”blue”
When the function getColor() is called in this example, the variable color is referenced. At that point,
a two - step search begins. First getColor() ’ s variable object is searched for an identifier named color .
When it isn ’ t found, the search goes to the next variable object (from the global context) and then
searches for an identifier named color . Because that variable object is where the variable is defined, the
search ends. Figure 4 - 5 illustrates this search process.
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