javascript

Chapter 4: Variables, Scope, and Memory
There is a deviation in the Internet Explorer (IE) implementation of JavaScript,
where the error caught in a catch statement is added to the execution context ’s
variable object, making it accessible even outside the catch block.
No Block - Level Scopes
JavaScript ’ s lack of block - level scopes is a common source of confusion. In other C - like languages, code
blocks enclosed by brackets have their own scope (more accurately described as their own execution
context in ECMAScript), allowing conditional definition of variables. For example, the following code
may not act as expected:
if (true) {
var color = “blue”;
}
alert(color);
//”blue”
Here, the variable color is defined inside an if statement. In languages such as C, C++, and Java, that
variable would be destroyed after the if statement is executed. In JavaScript, however, the variable
declaration adds a variable into the current execution context (the global context, in this case). This is
important to keep in mind when dealing with the for statement, which is typically written like this:
for (var i=0; i < 10; i++){
doSomething(i);
}
alert(i); //10
In languages with block - level scoping, the initialization part of the for statement defines variables that
exist only within the context of the loop. In JavaScript, the i variable is created by the for statement and
continues to exist outside the loop after the statement executes.
Variable Declaration
When a variable is declared using var , it is automatically added to the most immediate context
available. In a function, the most immediate one is the function ’ s local context; in a with statement, the
most immediate is the function context. If a variable is initialized without first being declared, it gets
added to the global context automatically, as in this example:
function add(num1, num2) {
var sum = num1 + num2;
return sum;
}
var result = add(10, 20); //30
alert(sum);
//causes an error since sum is not a valid variable
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