1. Scope of variables
The execution process of JavaScript script is divided into two stages:
In the first stage, the js engine () first scans the entire javascript code. When encountering <script>, a global active object will be created first, and the declaration of variables and function definitions appearing in <script> will be saved in the active object. If a function is encountered, a corresponding local active object is created, and the declaration of variables inside the function and the definition of the function are saved in the active object. </script>
In the second stage, when executing the javascript code, when encountering a variable, it will first search from the corresponding active object. If it cannot find it, it will find the upper-level active object.
<html>
<head>
<script>
var i = 100;
function f1(){
alert(i);
var i = 1000;
}
function f2(){
var i = 1000;
alert(i);
}
function f3(){
alert(i);
i = 1000;
}
function f4(){
alert(i);
i = 1000;
function f5(){
var i = 10000;
}
}
f1(); //结果是undefined
f2(); //结果是1000
f3(); //结果是100
</script>
</head>
<body>
</body>
<!--
预编译－－对脚本扫描
js引擎－－创建一个全局活动对象(i和f1)－－创建一个局部的活动对象(i)s
-->
</html>2. Math (an object built into javascript, which can be used directly)
Math.random(): Returns a random number between 0 and 1 0Math.ceil(): Round up
Math.floor(): Round down
<html>
<head>
<script>
function f1(){
var a1 = Math.random();
alert(a1);
var a2 = 100.68;
alert(Math.ceil(a2));
alert(Math.floor(a2));
}
function f2(){
Math.floor(Math.random()*33);
}
f1();
</script>
</head>
<body>
</body>
<html>3. W3C DOM model
1. What is dom
document object model
Converts a structured document (xml, html) into a tree and provides operations on the tree (including traversal, search, modification , delete, etc.) related attributes or methods
2. The basic structure of w3c dom model
Node DocumentHTMLDocument (<html>)HTMLBodyElement (<body>)ElementHTMLElementHTMLFormElement (<form>)HTMLInputElement (<input>)HTMLSelectElement (<select>)HTMLOptionElement (<option>)
The above is the content of Xiaoqiang’s HTML5 mobile development road (29) - JavaScript review 4, For more related content, please pay attention to the PHP Chinese website (www.php.cn)!
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