Notes on JS

Posted by inferium on Sat, 18 Sep 2021 12:00:30 +0200

——JS code should be written to the script tag


Control the browser to pop up a warning box
  alert("first JS code");

Let the computer output a content in the page
  document.write() can output a content to the body

Output a content console.log() to the console;

You can write js code into the onclick attribute of the tag
     When the button is clicked, the JS code will be executed

<button onclick="alert("dianyixia ");">Click on me</button>

You can write the JS code to the href attribute of the hyperlink. When you click the hyperlink, the JS code will be executed. Writing to tag properties is not recommended.

 <a href="javascript:alert('dianjiyixia')">You order me, too</a>
<a href="javascript:;">You order me, too!</a>

You can write JS code to an external JS file and import it through the script tag
     External writing can be referenced in different pages at the same time, or the browser's cache mechanism can be used

<script type="text/javascript" src="js/script.js"></script>
<script type="text/javascript">
	alert("I am script Code in label!");

  Once referenced, its internal code cannot be written. You need to create a new script tag to write code

JS is strictly case sensitive, and each statement ends with a semicolon

——Literal and variable

Literal quantities are unchangeable quantities, such as 1 2 3 4 5  

Variables are used to save literal quantities, and the value of variables can be changed arbitrarily. In development, variables are used to save a literal quantity

	// Declare variable
	// var
		var a;
	// Variable assignment
	// Declaration and assignment are performed simultaneously
		var b=789;
		var c=0;
		var age=80;

Identifiers (all that we can name independently in js can be called identifiers), such as variable name, function name, attribute name, etc. The identifier can contain letters, numbers, -, $. Identifier cannot start with a number. Cannot be a keyword or reserved word in ES. The identifier generally adopts hump naming method (i.e. the first letter is lowercase, and the first letter of each word is uppercase).

A data type is a literal type
         There are six data types in JS
             String string
             Number value
             Boolean Boolean
             Null null value
             Undefined undefined
             Object object
         The first five belong to the basic data type, and the Object belongs to the reference data type

In js, strings need quotation marks or single quotation marks
  Quotation marks cannot be nested, but single quotation marks can be placed in double quotation marks, and double quotation marks can be placed in single quotation marks.

      var str="hello";
	  str='He said:"it's a nice day today!"';
	  You can use \ as an escape character in a string
	  \' Represent '
	  \n Line feed
	  \t Tab
	 str="He said:\"it's a nice day today!\"";

Operators are also called operators
         Operators can operate on one or more values and obtain the operation results
         Arithmetic operator
         When values of non Number type are evaluated, these values are converted to Number and then evaluated
         NaN is required for any value and NaN operation
         You can add two values and return the result
         If you add two strings, the string will be spliced and returned
         Any value added with a string will be converted into a string and concatenated
         Any value will be automatically converted to Number when performing - * / operation
         You can convert it to Number by a value of - 0 * 1 / 1

Unary operator, only one operand is required
        + Plus sign   No impact on numbers
        - minus sign   You can reverse the negative sign of a number
         For values of type other than Number, they are converted to Number for further operation
          + Any data type = Number type

The value of a + + is the value before self increment,
       ++ The value of a is self increasing
       a -- the value of is the value before self subtraction
       -- The value of a is the value after subtraction

//Logical operator
        // && And
        // || or
        // ! wrong
        var a=true;
        // If you operate on a non Boolean value, it will be converted to a Boolean value for further operation
        // You can invert an arbitrary data type twice to convert it to a Boolean value. The principle is the same as that of the BOOlean() function
        var b=10;

// And operation: if the first value is TRUE, the second value is returned
		  If the first value is false,The first value is returned directly
 // Or operation: if the first value is TRUE, the first value is returned
		If the first value is false,The second value is returned directly
		// And operation: if both values are TRUE, the following is returned
		// trur && true
		var result=5 && 6;
		// If there is FALSE in the two values, the first FALSE is returned
		// false && false
		result=0 && 2;
		result=0 && 2;
		//false && false
		result=0 && NaN;
		result=NaN && 0;

===Congruent,! = = incomplete

		Conditional operators are also called ternary operators
		Syntax: conditional expression? Statement 1: Statement 2;
		When the conditional operator is executed, it first evaluates the conditional expression,
		If the value is TRUE, statement 1 is executed and the execution result is returned
		If the value is FALSE, statement 2 is executed and the execution result is returned
	   false ? alert("Statement 1") : alert("Statement 2");
	   var a=30;
	   var b=10;
	   a > b ? alert("a large") : alert("b large");
	   var max=a>b?a:b;

The priority above is high, and the same line is calculated from left to right. The priority can be changed through parentheses.


Topics: Javascript html5 html