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       var str1 = 'hello world';
        var p = str1.indexOf('l');
        console.log(p); //2
        p = str1.indexOf('l', 3);
        console.log(p); //3
        p = str1.indexOf('l', 4);
        console.log(p); //9
        p = str1.indexOf('llo');
        console.log(p); //2
        p = str1.indexOf('lloo');
        console.log(p); //-1

Be careful ： stay indexof() The parameters of the , It can be a single character , Find it and return to its first position , Go back if you can't find it -1.

2、charAt()

charAt() Method returns the specified character from a string .

The grammar is as follows ：str.charAt(index)

        var str = 'hello world';
        var c = str.charAt(7);
        console.log(c); //o
        c = str.charAt(-1);
        console.log(c); //  No output   Don't complain 
        c = str.charAt(20);
        console.log(c); //  No output   Don't complain

Be careful ： When the specified index value is not within the range of the string , No mistakes , But there will be no output .

3、concat()

concat() Method to join and merge one or more strings with the original string , Form a new string and return .

The grammar is as follows ： str.concat(str2, [, ...strN])

Example ：

        var str = 'hello';
        var s = str.concat(' world', '!');
        console.log(s); //hello world!

Be careful ：

（1） Use concat() To splice strings is the same as using the plus sign to splice strings , But use concat() A little more elegant .

（2） But it should be avoided in development , Used especially in loops （ Memory footprint ）.

4、substring()

substring() Method returns a subset of a string between the start index and the end index , Or from the beginning of the index until the character

A subset of the end of the string .

The grammar is as follows ：str.substring(indexStart[, indexEnd])

Example ：

        var str = 'charset';
        console.log(str);
        var s = str.substring(1); // harset  Intercept from subscript to 1 To the end of the string  
        console.log(s);
        s = str.substring(1, 5);
        console.log(s); //hars  Left closed right away 
        s = str.substring(1, 3);
        console.log(s); // ha
        s = str.substring(3, 1);
        console.log(s); // ha   be equal to s = str.substring(1, 3);
        s = str.substring(-1);
        console.log(s); // If the first argument given is a negative number , Then it will copy the string and return .

explain ：

（1） If only the first parameter is specified , Then the substring will run from the specified position to the end ;

（2）. If both parameters are specified , Then the substring will change from the start position to the character before the end position ;（ Left closed right away ）

（3） If the second parameter is less than the first parameter , Then it will intercept from the back to the front ; （ It is equivalent to the result of parameter exchange ）

（4） If the first argument given is a negative number , Then it will copy the string and return .

5、substr()

substr() Method returns a string intercepted at the start index position lenght individual .

The grammar is as follows ：str.substr(from , length);

Parameter description ：

（1）from, value type , It refers to the starting position of interception

（2） length, value type , Refers to the intercepted length

Example ：

        var str = 'charset';
        console.log(str);
        var s = str.substr(1, 2);
        console.log(s); //ha
        s = str.substr(4, 2);
        console.log(s); //se     From the index number to 4 Intercept 2 position 
        s = str.substr(2);
        console.log(s); // arset  Cut to the end  
        s = str.substr(-2);
        console.log(s); // et    Return to intercept

Be careful ：

（1） The parameter of this method is from that position , How many characters to intercept .

（2） If both parameters are specified , Then the character of the specified length is intercepted from the specified position to form a new word

Fu string .

（3） If only the first parameter is specified , Then it will intercept from the specified position to the end of the string to form a new character

strand .

（4） If only one parameter is specified , And this parameter is negative , Will be inverted to form a new string .

6、slice()

slice() Method to extract part of a string , And return a new string , And it doesn't change the original string .

The grammar is as follows ： str.slice(beginIndex[, endIndex])

Example ：

        var str = 'The morning is upon us.';
        console.log(str);
        var s = str.slice(5);
        console.log(s); //orning is upon us.
        s = str.slice(5, 9);
        console.log(s); //orni
        s = str.slice(5, -3);
        console.log(s); //orning is upon
        s = str.slice(-1, 5);
        console.log(s); // No output content

explain ：

（1） The function of this method and substr() Similar function of , Are used to intercept substrings .

（2） If only the first parameter is specified , From the specified position to the end of the original string .

（3） If you specify two parameters , Characters from the specified position to the end position will be extracted .

（4） If the second parameter is negative , The character from the specified position to the end of the number from the back to the front will be extracted .

（5） If the first argument is negative , There is no output , No mistake. .

7、replace()

replace() Method returns a replacement value （replacement） Replace some or all of the patterns （pattern） After match

New string of . A pattern can be a string or a regular expression , The replacement value can be a string or a string

Callback function to be called for every match . If pattern Is string , Replace only the first match . The original string will not be changed

change .

The grammar is as follows ：str.replace(regexp|substr, newSubStr|function)

Example ：

        var str = 'Twas the night before Xmas...';
        console.log(str);
        //  Put the letters  e  Replace with  , 
        var s = str.replace('e', ',');
        console.log(s); //Twas th, night before Xmas...   Only one was replaced 
        s = str.replace(/\s+/ig, '-');
        console.log(s); //Twas-the-night-before-Xmas...    Replaced all spaces

Be careful ：

（1）replace() Method has two parameters , The first parameter represents the character to look for , The second parameter represents the

character ;

（2）replace() Method can support regular expressions , stay JS The regular expression in is written in // Between , Such as

/\s+/ Means to match one or more spaces . After the regular expression , You can also add parameters , Such as ：i It means not distinguishing big

A lowercase letter ;g Represents a global match ;m Represents a match of multiple lines ;

（3） If it's a regular expression , You can't put this expression in quotation marks , If it is written in quotation marks, it will be regarded as a reference

Find a string to use .

8、replaceAll()

replaceAll() Method returns a replacement value （replacement） Replace some or all of the patterns （pattern） Matches

New string after . A pattern can be a string or a regular expression , The replacement value can be a string or a

Callback function to be called for each match . If pattern Is string , Replace only the first match . The original string will not

change .

The grammar is as follows ：str.replaceAll(regexp|substr, newSubStr|function)

Example ：

        var str = 'TwAs the night before Xmas...';
        console.log(str);
        var s = str.replaceAll('as', ',');
        console.log(s); //TwAs the night before Xm,...
        s = str.replaceAll(/\s+/g, '-');
        console.log(s); //TwAs-the-night-before-Xmas...
        s = str.replaceAll(/as/ig, ',');
        console.log(s); //Tw, the night before Xm,...

Be careful ：

（1）replaceAll() Method will replace all found characters ;

（2）replaceAll() Method if you want to use regular expressions , Then you must add a global match parameter (/g).

9、split()

split() Method uses the specified separator string to set a String Object is divided into substring arrays , With a specified split

String to determine the position of each split .

The grammar is as follows ：str.split([separator[, limit]])

Example ：

       var str = "Hello World How are you doing";
        console.log(str);
        var arr = str.split(' ');
        console.log(arr);
        arr = str.split(/\s+/);
        console.log(arr);

        
        str = "Hello,World,How,are,you,doing";
        console.log(str);
        arr = str.split(',');
        console.log(arr);

explain ：

（1）split() Method cuts the string according to the specified delimiter , Generate an array ;

（2）split() Method supports regular expressions to cut .

10、trim()

trim() Method to remove white space characters from both ends of a string . The white space characters in this context are all white space characters

And all line terminators .

The grammar is as follows ：str.trim()

Example ：

        var str = '    char set     ';
        console.log(str);
        var s = str.trim();
        console.log(s); //char set    There is no space before or after    The space in the middle cannot be removed

Be careful ： Only the front and back spaces can be deleted , The space in the middle cannot be removed .

11、match()

match() Method returns the result of a string matching the regular expression .

The grammar is as follows ：str.match(regexp)

Example ：

        var str = 'For more information, see Chapter 3.4.5.1';
        console.log(str); //  Define a regular expression 
        var regex = /see (chapter \d+(\.\d)*)/i;
        var flag = str.match(regex);
        console.log(flag);

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