Introduction to JavaScript

osc_09eh9rc8 2020-11-11 11:51:31
introduction javascript


know JavaScript

JavaScript Is a kind of script language belonging to the network , Has been widely used web application development , Often to add a variety of dynamic features to the web page , For users to provide a more smooth browsing effect . Usually JavaScript The script is embedded in the HTML To realize its own functions .

  • HTML Defines the content of a web page
  • CSS Rendering the layout and style of the web page
  • JavaScript Set the behavior of the web page

JavaScript It's a scripting language for browsers , At present, all browsers support JavaScript

JavaScript Is a lightweight object-oriented programming language

One thing I have to mention :JavaScript and java It doesn't matter !( It was originally called LiveScript, It's just that the heat was changed to JavaScript)

Easy to use JavaScript

stay HTML Use in JavaScript There are two ways :

  • stay HTML Use in < script>< /script> label , The embedded JavaScript Embedded script
  • stay HTML Use in < script src="" >< /script> label , Introduce the outside .js External chain of documents

Be careful : If the outer chain is used , In the label again js Code , Then the code inside the tag will not execute , If you still want to use inline in your page , Write another one. script Labels can be .script It's a double label , Even if you use the outer chain, there is no content in the tag , There should also be an end tag .

 <!-- Outer chain -->
<script src="../js/js_demo1.js"></script>
<!-- embedded -->
<script>
document.write("hello javaScript")
</script>

Rule of grammar

JavaScript Case sensitive

JavaScript It's a weakly typed language .

  • Weak type language : It's not that there is no type , It's about defining variables , There is no need to specify the specific data type

JavaScript Definition of variables :var Variable name = value

JavaScript Data types in :

  • Numeric type :number, For any number
  • Boolean type :boolean, It can be for true、false
  • String type :string, Both single and double quotation marks can declare strings , because JavaScript There is no character type
  • Reference type :object
    • Null value :null, Said empty , Belong to object type
    • undefined: Represents an uninitialized variable

Use typeof Can return the real type of the variable

// Defining variables 
var num1 = 100;
var num2 = 99.99;
var str = "haha";// Both single quotation marks and double quotation marks can be used to represent strings 
var flag = true;
document.write(num1+"<br>");
document.write("num1 The data type of is :"+typeof(num1)+"<br>");
document.write(num2+"<br>");
document.write("num2 The data type of is :"+typeof (num2)+"<br>")
document.write(str+"<br>");
document.write("str The data type of is :"+typeof str+"<br>")
document.write(flag+"<br>")
document.write("flag The data type of is :"+typeof flag+"<br>")

Three output modes

  • window.alert(""), Prompt box effect shows
  • console.log(""), Display in the console as a log
  • document.write(""), Display directly in HTML On the page
 var num=" Ha ha ha ";
// Output in the pop-up dialog box 
window.alert(num);
// Output on the page ( No matter script Where is the label , The output statements are all in body tag )
document.write(num);
// The output is in the browser console 
console.log(" Log information ");
console.warn(" Warning message ");
console.error(" error message ");

Knowledge point

There is a... In the operator === .

= Indicates assignment ,== Indicates that the comparison values are equal ,=== Indicates that the comparison type and value are equal

 var str = "100";
var num = 100;
document.write(" Use == Is it equal ?--"+(str==num))
document.write("<br> Use === Is it equal ?--"+(str===num))

In Boolean operations :

  • The number type is not 0 It's true
  • The string type is not ""( Empty string ) It's true
  • 0,null,undefined Are all false
 var num1 = 15;
if (num1){

console.log(" Really? ")
}
var str1 = "";
if (str1){

console.log(" Really? ")
}else{

console.log(" fake ")
}

function

JavaScript The function in is actually a method . But the format and java The method in is not the same

function  Function name ( parameter list ){

Method body
// Write the value return, If you don't, don't write 
}

characteristic :

  • There is no need to declare the return value type in the function
  • The parameters in the parameter list do not need to specify the data type
  • non-existent java The concept of overloading in , As long as two functions have the same function name , The previous function will be covered by the function
  • When using functions , If the function needs arguments , When calling, no parameter will be passed and no error will be reported , If the function doesn't need parameters , No error will be reported when the parameter is passed in

event

JavaScript It gives us the ability to create dynamic pages , Events can be JavaScript Detected behavior , When the event occurs , You can do something dynamically

Three elements of the event :

  • Event source : In the page , The elements that users can manipulate are event sources
  • event : User behavior with respect to the event source ( For example, mouse click 、 suspension ···)
  • Monitor : Used to listen for events , When the event occurs , The corresponding function will be called to handle the event

Events are usually used with functions , In this way, the function can be driven to execute by an event , In order to achieve some dynamic effects according to the user's behavior

Events can be browser behavior , It can also be user behavior

Common events :

Event name explain
onload A page is finished loading
onsubmit When the form is submitted ( The source of the event is form Forms )
onclick Mouse click on an object
onblur Double click on an object
onfocus Lose focus
onchange Focus of attention ( Mouse pointer over element , The element gets the focus )
onkeydown The content in the object is changed
onkeypress A keyboard key is pressed
onkeyup A keyboard key is pressed or held
onmousedown The mouse button is pressed
onmouseup Mouse button released
onmouseover The mouse is moved to an element
onmouseout Move the mouse out of an element
onmousemove Mouse moved

Example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>JavaScript event </title>
<script>
function clickMethod(){

window.alert(" You clicked the button ");
}
function loadMethod(){

window.alert(" The page is loaded ");
}
function changeMethod(){

window.alert(" You changed the contents of the text box ")
}
function focusMethod(){

window.alert(" The text box gets the focus ")
}
</script>
</head>
<body onload="loadMethod()">
<input type="submit" onclick="clickMethod()" value=" Button ">
<input type="text" onchange="changeMethod()" onfocus="focusMethod()">
</body>
</html>
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