例：截取love

var str = "I love javascript" ;
console.log(str.substr(2,4)); //love

例：截取love

var str = "I love javascript" ;
console.log(str.substring(2,6)); //love

var str = "Happy";
console.log(str.charAt(1)); //a

 var str = "0aA";
    console.log(str.charCodeAt(0)); //48(0)  根据索引查找字符对应的编码
    console.log(str.charCodeAt(1)); //97(a)
    console.log(str.charCodeAt(2)); //65(A)
    // a-z  97-122
    // A-Z 65-90
    // 0-9 48-57

 事件