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Learning content:
JavaScript official name ECMA script or ECMA-262
Short JS
1.js Embedding Method:
(a) JavaScript prefixes introduced into JS
1 < href= "Javascript:alert (' Hello World ')">Hello</a >(b) Inline references
Use the script tag, which can be placed between the head or body tag, loading order head>body, because the HTML file is loaded from top to bottom
1 <Head>2 <Script>3 Alert ("This is the second method of introduction")4 </Script>5 </Head>
(c) External references
External reference JS file, similar to the external reference of CSS, can also be placed between the head tag or body tag
1 < Body > 2 < src= "Jsd1.js"></script>4 </body>
2. Basic syntax
(a) The name of the variable: the first letter must be a letter (uppercase and lowercase), an underscore (_), or a dollar sign ($),
The remaining letters can be underlined, dollar signs, any letter or numeric characters
var x = B%log; Illegal Disclaimer:% not Available
Can't use keywords!
Keywords refer to words that have been defined by syntax as having special meanings, var = var; it's an illegal statement.
Variable declaration is case sensitive var x = 1 Not equal to var x = 1
(b) Declaring variables:
An implicit declaration such as x = 1
An explicit declaration such as var x = "Hello"
1 <script>2 x = 1; implicitly declares 3 var x = "Hello"; Explicit Declaration 4 </script>
<script> var y; // assign a value after declaring the variable first </script>
1 <script>2 var a = 1, b = 2, c = 3; declare multiple variables at once 3 </script>
3. Variable Common data types
(a) numeric type: Contains integral and floating-point types
(b) String type: must be enclosed in single or double quotation marks.
(c) Boolean type: True False
(d) Undefined type: specifically used to determine a variable that has been created but has no initial value, such as Var x; alert (x);
(e) null type var x = null;
(f) Object type array, date, and so on
Ps:
= is the assignment = = Determines whether the two ends are equal, only considering that the value does not consider the data type for example: Var x=2 var y= "2" x==y true
= = = Consider data value and type X===y false
In a strict sense, an empty type is not equal to an undefined type such as Var x; var y = null; X===y false
Null type is assigned a null value, undefined type not assigned value
Single-line comment//Multiline comment/**/
1<script>2 varb =NULL;//Empty Type3 varA = 10;//Integral type4 varc = 1.25;//floating Point Type5 vard = "Hello";//String Type6 varE =true;//Boolean type7 varF//type not defined8 varg = new Array (+/-);//Array Object Type9 varh = 3e2;//The method of integral type scientific countingTen</script>
2018/02/23
First day of JavaScript
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