Introduction to basic syntax of javaScript_basic knowledge

Introduction

JavaScript is a scripting language. (Scripts are text commands one by one. During execution, an interpreter of the system translates them one by one into machine-recognizable instructions and then executes them. Common scripts: batch scripts, T-SQL scripts, VBScript, etc. )

HTML is just a markup language that describes the appearance of a web page. It has no calculation or judgment capabilities. If all calculations and judgments (such as judging whether the text box is empty, judging whether the two passwords entered are consistent) are executed on the server side, the page will be It will be very slow, difficult to use, and puts a lot of pressure on the server. Therefore, it is required to be able to perform some simple operations and judgments in the browser. JavaScript is a scripting language that is executed on the browser side.

JavaScript has no direct relationship with Java. The only relationship is that JavaScript was originally called LiveScript. It later absorbed some features of Java and was upgraded to JavaScript. JavaScript is sometimes referred to as JS.

JavaScript is an interpreted language and can be run at any time without compilation.

JavaScript can be cross-platform, as long as there is a browser that supports JS. (Windows, Linux, Mac, Unix)

Grammar and precautions

1) Case sensitivity: JavaScript is strictly case sensitive. (n and N are two different variables.)

2) Weakly typed language, use var to declare variables: var num=10; num=true; OK

3) Use single quotes for strings: var msg=‘Hold on’

4) A semicolon after each sentence

5) Js comments are the same as C# and Java (//single-line comments (recommended), /* multi-line comments */ (there will be problems when using regular expressions)).

Dynamic Language

JS is a very flexible dynamic language. It is not as rigorous as static languages ​​such as C#. The JS completion function in development tools is only an auxiliary and suggestion.

Member calls that appear with "." may not work, and members that do not appear with "." may also be called, so don't worry about code problems just because "it doesn't appear at all". When writing js, you should know what members the current object has and cannot rely on smart prompts.

Dynamic languages: determine data types at runtime.

JavaScript is a dynamic type, so var i=10;i=‘variable’ is legal.

Reference external JS files

In addition to declaring JavaScript in the page, you can also write JavaScript into a separate js file and then introduce it into the page: .

The advantage of declaring to a separate js file is that multiple pages can be shared and reduce network traffic.

Note:

1) The <script> tag for importing external files can be written at the end of the document to improve user experience. </script>

2) There is no need to write <script></script>

in the js file

Variable naming rules

1) Starting with a letter, underscore or $,

2) The middle can include letters, numbers, underscores or $. (There is an extra $ in the variable naming)

3) It is recommended to use single quotes to declare strings

4) It is recommended to declare local variables with var

Determine whether it has been declared

How to determine whether declared variables and parameters are initialized (available) in JavaScript:
Assume there is already a variable x:

Copy code The code is as follows:

if (typeof(x) !=‘undefined’ && x!=null) { alert("Available"); }
if(x) { alert('Variable is available!'); } else { alert('Variable is not available!'); } //null, undefined, '', and 0 are all considered false

Recommend the second one

Block-level scope does not exist

Variables declared in a for, while, or if block inside a function also have scope within the entire function, so in order to avoid confusion, it is better to declare these variables directly at the beginning of the function.

Copy code The code is as follows:

Var n=10;
function ff(){
n ;
};

Conclusion: There is no block-level scope in JS.