Day28 strict mode, string JS 2021.09.22

ES5 The strict model of ： Use the syntax ： "use strict";

Instructions ：

-- Put it at the top of the code , That is to say script The first line in a tag or a function , Otherwise it will fail

-- Strict mode can regulate the current script Tag or code in the current function , Can not regulate other script The code in the tag and other functions

Standard grammar ：

1. Implicit declaration of variables... Is not allowed

2. It is forbidden to duplicate the names of formal parameters in functions

3. Disable in function this representative window

Why use strict patterns ？

1. It can make the code more standardized

2. It can make the code run faster , Improve operational efficiency

Be careful ： When merging files , The strict mode code in the first line is invalid , It is recommended that the package be in a self executing function

character string ：

Through the asker code table , We can get some rules of string comparison ：

-- Letters are larger than numbers

-- Lowercase letters are larger than uppercase letters

-- The letters get bigger as they come back

Common methods of string ：

 charAt( Subscript )： Get the corresponding characters in the string by subscript

- The string is also subscripted It can be done by string [ Subscript ] Get the corresponding characters

- Strings are just readable States , You cannot modify or add

- Strings can also be looped

charCodeAt( Subscript )： Get the corresponding character by subscript ASCII

String.fromCharCode(unicode code )： adopt ASCII Get character , Multiple encodings are separated by commas .

split( Segmentation symbols )： According to the separator 、 Split the string into an array .

- split In parentheses, except for the division symbol , You can also separate them with commas followed by a parameter number type It means to keep several data

substring( The starting subscript , Ending subscript )： Used to intercept the contents of a string

- Start subscript contains The ending subscript does not contain

- If the end subscript of the second parameter is not written It means that all characters from the beginning to the end will be intercepted

- When the start subscript and the end subscript are equal, it will return ' empty '

- When the start subscript is greater than the end subscript , The two parameters will be exchanged and intercepted

- If the subscript is negative , The default conversion is 0 Then intercept

substr(start, length)： Used to intercept the contents of a string

- The number of ends can also be omitted Omission means that everything from the current subscript to the following is intercepted

slice( Start subscript , End subscript )： The way to intercept a string , If it's negative , Intercept from back to front , Include the starting index .

- Start subscript contains The ending subscript does not contain

- The second parameter is optional If you don't write, it means to intercept to the end

- Both parameters can be negative numbers

- The first parameter is greater than the second parameter Returns an empty

- The first parameter is less than the second parameter

indexOf() The position of the first occurrence of a character

laseIndexOf() The last position of the character

concat( )： Consistent with array method .

grammar : character string .concat( Spliced string )

toUpperCase()： Convert to uppercase .

toLowerCase()： Convert to lowercase .