JavaScript基础 语言特性

JavaScript是基于函数的语言，一切都是对象，但又比较特殊

引用	引用是指向实际对象的一个指针和C/C++的指针一样,C#和java的对象也是引用传递 函数重载 // 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 );
作用域	作用域是由函数划分的，而不是块(Block)划分，和其他的语言不一样的地方 // 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全局作用域就是window对象 // 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" );
闭包	闭包意味着内层的函数可以引用存在于包含它的函数内的变量，即使外层函数的执行已经终止 详细参考：http://jibbering.com/faq/faq_notes/closures.html 不用产生全局函数： // 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 这个功能在JavaScript中充分发挥 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 ); 以上处理有些不太同一理解，JavaScript提供了call和apply方法实现这个功能： // 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" );