JavaScript基礎 – 語言特性

JavaScript是基於函數的語言，一切都是對象，但又比較特殊

引用	引用是指向實際對象的一個指標和C/C++的指標一樣,C#和java的對象也是引用傳遞   函數重載 // A simple function for sending a 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 calls work sendMessage( "Hello, World!" ); sendMessage( "How are you?", window );
範圍	範圍是由函數劃分的，而不是塊(Block)劃分，和其他的語言不一樣的地方 // 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" );   JavaScript全域範圍就是window對象 // A globally-scope variable, containing the string 'test' var test = "test";   // You'll notice that our 'global' variable and the the // property of the 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" );
閉包	閉包意味著內層的函數可以引用存在於包含它的函數內的變數，即使外層函數的執行已經終止 詳細參考：http://jibbering.com/faq/faq_notes/closures.html 不用產生全域函數： // Create a new anonymous function, to use as a wrapper (function(){     // The variable that would, normally, be global     var msg = "Thanks for visiting!";       // Binding a new function to a global object     window.onunload = function(){         // Which uses the 'hidden' variable         alert( msg );     };   // Close off the anonymous function and execute it }());
上下文	this 這個功能在JavaScript中充分發揮   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 );   以上處理有些不太同一理解，JavaScript提供了call和apply方法實現這個功能： // 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" );