<!DOCTYPE html>
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<meta charset="UTF-8">
<title>Document</title>
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<script>
//concat()：连接两个字符串返回一个新字符串，并且不会被修改
var str1 = "hello";
var str2 = "123";
var str3 = str1.concat(str2);
console.log(str1);
console.log(str2);
console.log(str3);
var str = "I love my family!";
console.log(str);
//slice()：根据索引值和索引值截取字符串
console.log(str.slice(2)); //从索引截取到最后
console.log(str.slice(2,5)); //从索引截取，包左不包右
console.log(str.slice(-3)); //倒数几个
console.log(str.slice(5,3)); //空字符串
//substr()：根据索引值和长度截取字符串
console.log(str.substr(2)); //从索引截取到最后
console.log(str.substr(2,6)); //从索引截~长度个字符串
console.log(str.substr(-3)); //倒数几个
//substring()：根据索引和索引值截取字符串
console.log(str.substring(2)); //从索引截取到最后
console.log(str.substring(2,5)); //从索引截取~长度个字符串
console.log(str.substring(-1)); //全部截取
console.log(str.substring(5,2)); //只能调换
</script>
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