Problems with converting numbers and strings to each other in JS-JS Tutorial-php.cn

Problems with converting numbers and strings to each other in JS

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Release： 2017-10-20 09:42:24

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Convert string to number
1parseInt() function

parseInt() 函数从string的开始解析，返回一个整数
parseInt(&#39;123&#39;) : 返回 123；
parseInt(&#39;1234xxx&#39;) : 返回 1234；
parseInt(&#39;123.456&#39;) : 返回 123；
parseInt(&#39;1 2 3&#39;) : 返回 1；字符串的情况，自会返回第一个数
parseInt(&#39;bb cc 12&#39;) : 返回 NaN；字符串第一个不是数，返回nan
parseInt(&#39;123&#39; 321) : 返回 321；
parseInt("AF", 16); 返回 175；会自动把二进制十六进制八进制的转化成数字
如果解析不到数字，则将返回一个NaN的值，可以用isNaN()函数来检测；
parseFloat()只会返回小数

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2 Add " before +”

var n =+"123":返回 123

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3 characters plus “*”

var n ="123"*1:返回 123

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4 string array splitting added

var arr = str.split(",") 一个字符串组，用，都分割成多个
b = a.join("-");-是指定的分割符，可以随意，将数组转化成一个字符串

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5 Adding strings and numbers returns a string

    console.log(12+"12"):返回"1212"
   console.log(&#39;12&#39; + &#39;34&#39;)返回&#39;1234&#39; 
   console.log(&#39;12&#39; + 34 ) 返回&#39;1234&#39; 
   console.log(12 + &#39;34&#39;&#39;）返回1234&#39; 
   console.log(12 + 34 )  返回46

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6 When adding strings and multiple numbers, use Expand the brackets

  ("<img src=&#39;images/" + 2 + 1 + ".jpg&#39;>") 返回("<img src=&#39;images/" + 21 + ".jpg&#39;>") 
   如果想转成3
   ("<img src=&#39;images/" + （2+1）+ ".jpg&#39;>") 返回("<img src=&#39;images/" + 3 + ".jpg&#39;>") 
   如果前面是数字相加，则可以先把数字加起来
   var a = 10 + 20 + &#39;abc&#39; + &#39;cd&#39;; 返回a的值为:30abccd

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7eval()If you want to perform string operations

eval("x=10;y=20;document.write(x*y)")    返回200
document.write(eval("2+2"))　　　　　　返回4
var x=10
document.write(eval(x+17）　　　　　　返回27

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Number to string

1tostring（）
   n = 100
   x = n.toString() => "100"或是（100）toString()       //tostring（2/16/8）还可以实现进制的转化
2数字+任意字符串“”
   var n = 1234;
   var nn = 1234+""

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About string

Judgement Whether the string contains a string that contains the return subscript and does not contain the string that returns -1

var i = str.indexOf("x")

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The last subscript

var i = str .lastIndexOf("")

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Convert to uppercase and lowercase

　str.toUpperCase()  转大写
　　str.tolowerCase()  转小写

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Intercept string string.substr ("Start from", "Intercept a few")

　str.substr(0,5)
<body>

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