javascipt scope

From Apress ProJavaScriptTechniques

In JavaScript, scope is kept within functions, but not within blocks (such as while, if, and for statements).

js 代码

// Set a global variable, foo, equal to test
var foo = "test";
// Within an if block
if ( true ) {
// Set foo equal to 'new test'
// NOTE: This is still within the global scope!
var foo = "new test";
}
// As we can see here, as foo is now equal to 'new test'
alert( foo == "new test" );
// Create a function that will modify the variable foo
function test() {
var foo = "old test";
}
// However, when called, 'foo' remains within the scope
// of the function
test();
// Which is confirmed, as foo is still equal to 'new test'
alert( foo == "new test" );

An interesting aspect of browser-based JavaScript is that all globally scoped variables are actually just properties
of the window object.

js 代码

// A globally-scoped variable, containing the string 'test'
var test = "test";
// You'll notice that our 'global' variable and the test
// property of the the window object are identical
alert( window.test == test );

In Listing 2-12 a value is assigned to a variable (foo) within the scope of the test() function. However, nowhere in Listing 2-12 is the scope of the variable actually declared (using var foo). When the foo variable isn’t explicitly defined, it will become defined globally, even though it is only used within the context of the function scope.

Listing 2-12

js 代码

// A function in which the value of foo is set
function test() {
foo = "test";
}
// Call the function to set the value of foo
test();
// We see that foo is now globally scoped
alert( window.foo == "test" );