JS defines the basic knowledge of variables and data types

Web API includes DOM operation documents, such as moving, size, adding and deleting page elements. BOM operation browser, such as page pop-up window, detecting window width, storing data to browser, etc

2, Writing position of JS

1. Inline

	<h2 onclick="alert('Click')">title</h2>

2. Embedded

<body>
	<script>
		// JS code is written here
		// Pop up warning window
		alert('It will pop up here');
	</script>
</body>

3. External chain

	<!-- Note: if you want to use src External link a JS File, then script Never write code in the label -->
	<script type="text/javascript" src="index.js"></script>

3, Input / output statement

If you want to see something in js, you must use input and output statements

1. Enter a statement

The first quotation mark is the prompt information, and the second quotation mark is the default input

prompt('Please enter gender', 'male');

effect:

2. Output statement

sentence	explain
document.write('<h1>666</h1>');	You can display the content in the body
alert('pop up here ');	Pop up warning box
console.log('the console will print this string ');	Print on console

document.write('<h1>666</h1>');

effect:

 alert( 'Pop up here' );

console.log('The console will print this string');

4, Definition and use of variables

1. Define variables

let	Valid in local variable {}
var	global variable

let and var define variables differently:

let defined variables are not allowed to be redefined, while var defined variables can be redefined

let defined variables have block level scope, while var defined variables do not have block level scope

Pre resolution:

let defines the variable code

		let phone;// Variable name
		phone = 13312341234;
		document.write(phone);

2. Notes on variables

2.1 variables defined with let cannot be redefined and can be changed

		let uname = 'Spider-Man';
		uname = 'Iron Man';
		uname = 'Ultraman';

2.2 if we want to use more variables, we can define multiple variables at the same time

let a,b,c;

2.3 if a variable is only defined but no data is saved, it is undefined by default

		let age;
		console.log(age);

effect:

3. Variable naming convention

Rules (must not be violated)

Variable names must not be keywords
Variable names can contain numbers, letters, underscores$
Variable names cannot start with numbers
Variable names cannot contain spaces
Variable names are case sensitive

Specification (recommended)

It is suggested that the variable name should be meaningful
Hump nomenclature For example: userNameSex:

5, Array

Because variables can only save one data at a time, it is inconvenient to save multiple data

Array: used to save a combination of multiple data. let variable name = [value 1, value 2, value 3, value 4, value 5,...];

Need to use Variable name [subscript] The subscripts of value 1, value 2, value 3, value 4, value 5,... Are 0, 1, 2, 3, 4

		let arr = ['Fei Zhang', 'Guan Yu', 'Zhao Yun', 'Zhang Liao', 'army officer's hat ornaments'];
	    console.log( arr[0] );
		console.log( arr[1] );

effect:

Any array has an attribute: length, which is used to view the length of the array (the number of arrays)

		let arr = ['aa', 'bb', 'cc'];
		console.log(arr.length);

effect:

6, Data type

Number type	number
Character type	Chinese characters, letters and symbols
Boolean type	ture,fasle
undefined type	undefined object is null
null type	null means that the variable is empty

1. Number type

The 'Number' type is also a Number type

		let age = 23;
		let pi = 3.14;
		let num = 2;

2. Character type

It is usually wrapped in quotation marks, such as Chinese characters, letters and symbols

		let uname = 'Delireba';
		let userName =  'wahaha';
		let str = 'study hard and make progress every day.?: !*';

String splicing

If the variable is wrapped in quotation marks, it becomes an ordinary string. The string cannot be written directly next to anything else. In the string, there is a splicing operation of + identification string

		let uname = 'Iron Man';
		let age = 46;
		document.write('My name is' + uname + '，this year' + age + 'year');

effect:

Template string:

If you want to identify variables: just ${variables}

		let uname = 'A Fei';
		let age = 22;
		let sex = 'male'


		document.write(`<ul>
							<li>${uname}</li>
							<li>${age}</li>
							<li>${sex}</li>
						</ul>`);

effect:

3. Boolean type

Describes the type of true and false values. There are only two values, true (true) and false (false)

4. undefined type

If a variable is defined without assignment, it is undefined, which means that the variable is empty

		let num;
		console.log(num);

5. null type

Indicates that the variable is empty

7, Display conversion

1. To numeric type

let n = '26';
n=Number(n);

If the string contains non numeric characters, the result of conversion is NaN

let n = '26a';
let m = Number(n);
console.log(m);

effect:

What happens when there are numbers in the string that need to be taken out?

parseInt:	Rounding, (take out the integer part)
parseFloat:	Take floating point number, (take floating point number)

parseInt: rounding, (take out the integer part)

		let n = '26.67.34a';
		//parseInt: rounding, (take out the integer part)
		let re = parseInt( n )
		console.log( re );

effect:

parseFloat: floating point number, (floating point number)

		let n = '26.67.34a';
		// parseFloat: floating point number, (floating point number)
		let re = parseFloat(n)
		console.log(re);

effect:

2. To string

String (variable name)
Variable name. toString();	toString is not allowed for null and undefined

String to string

		let num = 123;
		let re = String( num);
		console.log( re );

Variable name. toString();

		let num = 123;
		let re = num.toString(); // toString is not allowed for null and undefined
		console.log( re );

3. To boolean type

Use Boolean (variable name) to convert variables to Boolean types. Since Boolean types only have true and false, except

0, '', null, undefined and NaN will be converted to false, and the rest will be converted to true

Posted by kidd1270 on Sat, 20 Nov 2021 11:37:17 -0800

Programmer Group

JS defines the basic knowledge of variables and data types

Article catalog

preface

1, What is JavaScript?

2, Writing position of JS

1. Inline

2. Embedded

3. External chain

3, Input / output statement

1. Enter a statement

2. Output statement

4, Definition and use of variables

1. Define variables

2. Notes on variables

3. Variable naming convention

Rules (must not be violated)

Specification (recommended)

5, Array

6, Data type

1. Number type

2. Character type

3. Boolean type

4. undefined type

5. null type

7, Display conversion

1. To numeric type

2. To string

3. To boolean type

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