如何用javascript从最后一行中删除某些字符串？

var str = "第一行\n第二行\n第三行\n最后一行";
var lines = str.split("\n");

var lastLine = lines.pop();

var searchString = "某些字符串";
var modifiedLastLine = lastLine.replace(searchString, "");

lines.push(modifiedLastLine);
var modifiedStr = lines.join("\n");

var str = "第一行\n第二行\n第三行\n最后一行";
var lines = str.split("\n");
var lastLine = lines.pop();
var searchString = "某些字符串";
var modifiedLastLine = lastLine.replace(searchString, "");
lines.push(modifiedLastLine);
var modifiedStr = lines.join("\n");

var str = "第一行\n第二行\n第三行\n最后一行123456";
var lines = str.split("\n");
var lastLine = lines.pop();
var modifiedLastLine = lastLine.replace(/\d/g, "");
lines.push(modifiedLastLine);
var modifiedStr = lines.join("\n");