ECMAScript

Web APIs

Web page special effects (monitoring some of the user’s behaviors to allow the web page to provide corresponding feedback)

Form validation (judging the legality of form data)

Data interaction (obtaining background data and rendering to the front end)

Server-side programming (node.js)

Write directly in the html file and wrap it with script tags

Specification: The script tag is written above </body>

Extension: alert("Hello, js") page pops up a warning dialog box

The code is written in a file ending with .js

Syntax: introduced into the HTML page through script tags

Single line comments

block comments

Function: Use English; represents the end of the sentence

Actual situation: In actual development, it can be written or not, and the browser can automatically infer the end position of the statement (without adding it)

document.write('content to be output')

alert("output content")

console.log("Console printing")

prompt("Please enter your name:")

used to store data

To use a variable, you first need to create it (also known as declaring a variable or defining a variable)

Syntax: let variable name

After defining a variable, you can initialize it. Follow the variable name with ‘=’ and then the value (Note: Get the data in the variable through the variable name)

You can also complete the assignment operation directly when declaring the variable. This operation is also called variable initialization.

After a variable is assigned a value, you can also update it by simply giving it a different value (note: let does not allow multiple declarations of a variable)

Variable case: exchanging the value of a variable

After a variable is assigned a value, you can also update it by simply giving it a different value.

Syntax: Multiple variables separated by commas

Note: It may appear that the code length is shorter, but this is not recommended. For better readability, please declare only one variable per line

Keywords cannot be used

It can only be composed of underscores, letters, numbers, and $, and numbers cannot begin with it.

Letters are strictly case-sensitive. For example, Age and age are different variables.

The name should be meaningful

Comply with CamelCase nomenclature: the first letter of the first word is lowercase, and the first letter of each subsequent word is capitalized, such as: userName

You can use it first and then declare it (unreasonable)

Variables declared with var can be declared repeatedly (unreasonable)

For example, variable promotion, global variables, no block-level scope, etc.

let array name = [data 1, data 2...data n]

Arrays are saved in order, so each data has its own number

Arrays can store any type of data

array name[subscript]

Element: Each data stored in the array is called an array element.

Subscript: the number of the data in the array

Length: The number of data in the array, obtained through the length attribute of the array

number numeric type

string string type

boolean Boolean type

undefined undefined type

null empty type

Detect data type

object object

As long as one of the two sides of the number is a string, the other will be converted to a string.

Arithmetic operators other than -*/ will convert data into numeric types.

The conversion type is not clear and can only be summarized based on experience.

The sign can be converted into a numeric type when parsed as a positive sign.

The result of adding any data to a string is a string

Number(data)

parselnt(data)

parseFloat(data)