JavaScript method to delete or extract specified characters from a string (very commonly used)

let a = "1,2,3";
document.write(a.substring(2,a.Length))
//The first parameter starts from the character with subscript 0, including the current subscript 0,
//The second parameter ends at the character with the subscript, excluding the current subscript. By default, it is +1 longer than the subscript position of the last character of the substring to be extracted in the string
 
//Truncate from the second subscript to the total length + 1, so the result output is: 2,3

var str="Hello!";
var n=str.substr(2,3)
// Extract 3 characters from the character with subscript 2, and the output result is: llo

var str="runab site";
var n=str.indexOf("a");
//If only one parameter is specified, the query will start from the beginning, specify the string value to be searched, and return the specified index after the query //The second parameter specifies the position to start searching in the string (including the input index), and returns the specified index after the query //The output result is: 3

var str="runab site";
var n=str.lastIndexOf("a",3);
//If only one parameter is specified, the query will start from the end and go forward. The specified index will be returned after the query. //The second parameter is the character with the subscript number to query forward (including the input subscript). The specified index will be returned after the query. //The output result is: 3
 
 
//Combined with the above (return the last character)
let str = str.substring(0, str.lastIndexOf('e'));

var str="Hello everyone!";
var n=str.replace("big","small");
//The output result is: Hello, little family!
 
 
//You can also use regular expression to replace var str="everyone is so big";
var n=str.replace(/大/g,"小"); //g is global replacement //output result is: 小家好小 //set the second parameter to empty to delete the string var str="大家好大";
var n=str.replace(/大/g,""); //g is a global replacement //The output result is: 家好

var str="hello world!"
var items = str.split("ll") // ["he", "oWorld!"]
//You will get an array, the items array includes multiple strings separated by ll (excluding ll)
var newStr=items.join(""); // heoWorld!
//join() uses an empty string to connect the arrays in the array into a new string. Without quotes, commas are used as separators by default.