如何在特定角色的最后一次出现之前插入一些东西？
if(statement)在最后一个“<”之前将“again”插入字符串;
解决方法
您可以将lastIndexOf()与substring()一起使用：
var str="Hello planet earth,you are a great planet.";
var n=str.lastIndexOf("planet");
var str2 = str.substring(0,n)+" again "+str.substring(n);
console.log(str2); // Hello planet earth,you are a great again planet.
作为一个很好的功能：
function insertBeforeLastOccurrence(strToSearch,strToFind,strToInsert) {
var n = strToSearch.lastIndexOf(strToFind);
if (n < 0) return strToSearch;
return strToSearch.substring(0,n) + strToInsert + strToSearch.substring(n);
}
var str ="This <br> is another <br> string <br> example.";
var newStr = insertBeforeLastOccurrence(str,"<"," again");
console.log(newStr); // This <br> is another <br> string again<br> example.
或者作为String方法：
String.prototype.insertBeforeLastOccurrence = function(strToFind,strToInsert) {
var n = this.lastIndexOf(strToFind);
if (n < 0) return this.toString();
return this.substring(0,n) + strToInsert + this.substring(n);
}
var str ="This <br> is another <br> string <br> example.";
console.log(str.insertBeforeLastOccurrence("<"," again"));
// Output: This <br> is another <br> string again<br> example.
console.log(str.insertBeforeLastOccurrence("w00t"," again")); // wont find
// Output: This <br> is another <br> string <br> example.
