Javascript basics-language features

Javascript is a function-based language. Everything is an object, but it is quite special

reference	A reference is a pointer to an actual object, which is the same as a pointer to a C/C ++ object, C # and Java objects are also references function overloading // a simple function for sending message function sendmessage (MSG, OBJ) { // if both a message and an object are provided If (arguments. length = 2) // send the message to the object obj. alert (MSG); // otherwise, assume that only a message was Provided else // so just display the default error message alert (MSG ); } // both of these function callwork sendmessage (" hello, world! "); sendmessage (" how are you? ", Window);
Scope	The scope is divided by functions, not blocks. It is different from other languages. // 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 ");   JavascriptThe global scope is window.Object // A globally-scope variable, containing the string 'test' VaR test = "test ";   // You'll notice that our 'global' variable and // Property of the window object are identical Alert (window. test = test );   // 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 ");
Closure	The closure means that the inner function can reference the variables in the function that contains it, even if the execution of the outer function has been terminated. Http://jibbering.com/faq/faq_notes/closures.html No need to generate global functions: // Create a new anonymous function, to use as a wrapper (Function (){ // The variable that wowould, normally, be global VaR MSG = "thanks for visiting! ";   // Binding a new function to a Global Object Window. onUnload = function (){ // Which uses the 'hidd' variable Alert (MSG ); };   // Close off the anonymous function and execute it }());
Context	This function is fully used in JavaScript.   VaR OBJ = { Yes: function (){ // This = OBJ This. Val = true; }, No: function (){ This. Val = false; } };   // We see that there is no Val property in the 'obj 'object Alert (obj. Val = NULL );   // We run the Yes function and it changes the Val Property // Associated with the 'obj 'object OBJ. Yes (); Alert (obj. Val = true );   // However, we now point window. No to the obj. No method and run it Window. No = obj. No; Window. No ();   // This results in the OBJ object staying the same (as the context was // Switched to the window object) Alert (obj. Val = true ); // And window Val property getting updated. Alert (window. Val = false );   Some of the above processes are not quite the same. Javascript provides the call and apply methods to implement this function: // A simple that sets the color style of its context Function changecolor (color ){ This. style. Color = color; } // Calling it on the window object, which fails, since it doesn' t // Have a style object Changecolor ("white ");   // Find the element with an ID of main VaR main = Document. getelementbyid ("Main "); // Set its color to black, using the call Method // The call method sets the context with the first argument // And passes all the other arguments as arguments to the Function Changecolor. Call (main, "black ");   // A function that sets the color on the Body element Function setbodycolor (){ // The apply method sets the context to the body Element // With the first argument, the second argument is an array // Of arguments that gets passed to the Function Changecolor. Apply (document. Body, arguments ); } // Set the background color of the body to black setbodycolor ("black");