很多时候，觉得自己掌握的东西还是很少，做事情的效率还不是很高，希望自己能够成长起来，认认真真做事。
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Javascript声明变量的时候，虽然用var关键字声明和不用关键字声明，很多时候运行并没有问题，但是这两种方式还是有区别的。可以正常运行的代码并不代表是合适的代码。
var num = 1；
是在当前域中声明变量. 如果在方法中声明，则为局部变量（local variable）；如果是在全局域中声明，则为全局变量。
而 num = 1；
事实上是对属性赋值操作。首先，它会尝试在当前作用域链（如在方法中声明，则当前作用域链代表全局作用域和方法局部作用域etc。。。）中解析 num； 如果在任何当前作用域链中找到num，则会执行对num属性赋值； 如果没有找到num，它才会在全局对象（即当前作用域链的最顶层对象，如window对象）中创造num属性并赋值。
注意！它并不是声明了一个全局变量，而是创建了一个全局对象的属性。
即便如此，可能你还是很难明白“变量声明”跟“创建对象属性”在这里的区别。事实上，Javascript的变量声明、创建属性以及每个Javascript中的每个属性都有一定的标志说明它们的属性----如只读（ReadOnly）不可枚举（DontEnum）不可删除（DontDelete）等等。
由于变量声明自带不可删除属性，比较var num = 1 跟 num = 1，前者是变量声明，带不可删除属性，因此无法被删除；后者为全局变量的一个属性，因此可以从全局变量中删除。
具体见以下代码：
// num1为全局变量，num2为window的一个属性
var num1 = 1;
num2 = 2;
// delete num1; 无法删除
// delete num2; 删除
function model(){
var num1 = 1; // 本地变量
num2 = 2; // window的属性
// 匿名函数
(function(){
var num = 1; // 本地变量
num1 = 2; // 继承作用域（闭包）
num3 = 3; // window的属性
}())
}
PS. 在ECMAScript5标准中，有一种“严格模式”（Strict Mode）。在严格模式中，为未声明的标识符赋值将会抛引用错误，因此可以防止意外的全局变量属性的创造。目前一些浏览器的新版本已经支持。
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下面收集一些基础的知识，有时候基础的东西不能忘记，这些都是我们今天得以站立的根基和基石。
The HTML DOM Document Object
HTML DOM Nodes
In the HTML DOM (Document Object Model), everything is a node:
- The document itself is a document node
- All HTML elements are element nodes
- All HTML attributes are attribute nodes
- Text inside HTML elements are text nodes
- Comments are comment nodes
The Document Object
When an HTML document is loaded into a web browser, it becomes a document object.
The document object is the root node of the HTML document and the "owner" of all other nodes:
(element nodes, text nodes, attribute nodes, and comment nodes).
The document object provides properties and methods to access all node objects, from within JavaScript.
Tip: The document is a part of the Window object and can be accessed as window.document.
Document Object Properties and Methods
The following properties and methods can be used on HTML documents:
| Property / Method | Description |
|---|---|
| document.activeElement | Returns the currently focused element in the document |
| document.addEventListener() | Attaches an event handler to the document |
| document.adoptNode() | Adopts a node from another document |
| document.anchors | Returns a collection of all <a> elements in the document that have a name attribute |
| document.applets | Returns a collection of all <applet> elements in the document |
| document.baseURI | Returns the absolute base URI of a document |
| document.body | Sets or returns the document's body (the <body> element) |
| document.close() | Closes the output stream previously opened with document.open() |
| document.cookie | Returns all name/value pairs of cookies in the document |
| document.charset | Deprecated. Use document.characterSet instead. Returns the character encoding for the document |
| document.characterSet | Returns the character encoding for the document |
| document.createAttribute() | Creates an attribute node |
| document.createComment() | Creates a Comment node with the specified text |
| document.createDocumentFragment() | Creates an empty DocumentFragment node |
| document.createElement() | Creates an Element node |
| document.createTextNode() | Creates a Text node |
| document.doctype | Returns the Document Type Declaration associated with the document |
| document.documentElement | Returns the Document Element of the document (the <html> element) |
| document.documentMode | Returns the mode used by the browser to render the document |
| document.documentURI | Sets or returns the location of the document |
| document.domain | Returns the domain name of the server that loaded the document |
| document.domConfig | Obsolete. Returns the DOM configuration of the document |
| document.embeds | Returns a collection of all <embed> elements the document |
| document.forms | Returns a collection of all <form> elements in the document |
| document.getElementById() | Returns the element that has the ID attribute with the specified value |
| document.getElementsByClassName() | Returns a NodeList containing all elements with the specified class name |
| document.getElementsByName() | Returns a NodeList containing all elements with a specified name |
| document.getElementsByTagName() | Returns a NodeList containing all elements with the specified tag name |
| document.hasFocus() | Returns a Boolean value indicating whether the document has focus |
| document.head | Returns the <head> element of the document |
| document.images | Returns a collection of all <img> elements in the document |
| document.implementation | Returns the DOMImplementation object that handles this document |
| document.importNode() | Imports a node from another document |
| document.inputEncoding | Returns the encoding, character set, used for the document |
| document.lastModified | Returns the date and time the document was last modified |
| document.links | Returns a collection of all <a> and <area> elements in the document that have a href attribute |
| document.normalize() | Removes empty Text nodes, and joins adjacent nodes |
| document.normalizeDocument() | Removes empty Text nodes, and joins adjacent nodes |
| document.open() | Opens an HTML output stream to collect output from document.write() |
| document.querySelector() | Returns the first element that matches a specified CSS selector(s) in the document |
| document.querySelectorAll() | Returns a static NodeList containing all elements that matches a specified CSS selector(s) in the document |
| document.readyState | Returns the (loading) status of the document |
| document.referrer | Returns the URL of the document that loaded the current document |
| document.removeEventListener() | Removes an event handler from the document (that has been attached with the addEventListener() method) |
| document.renameNode() | Renames the specified node |
| document.scripts | Returns a collection of <script> elements in the document |
| document.strictErrorChecking | Sets or returns whether error-checking is enforced or not |
| document.title | Sets or returns the title of the document |
| document.URL | Returns the full URL of the HTML document |
| document.write() | Writes HTML expressions or JavaScript code to a document |
| document.writeln() | Same as write(), but adds a newline character after each statement |