日常眼瞎之：freecodecamp 用“+=”连接字符串

题目要求是这个样子的：
1.myStr should have a value of This is the first sentence. This is the second sentence.
2.Use the += operator to build myStr;

我的代码是这个样子的：
 var myStr = "This is the first sentence.";
 myStr += "This is the second sentence.";

然鹅有问题，没有完成要求一，百思不得其解，于是请教度娘，看到了别人的代码：
var myStr="This is the first sentence.";
myStr += " This is the second sentence.";

我觉得一毛一样，还是百思不得其解，于是一句一句看（总共才两句......）
终于发现This is the second sentence前面有个空格......一把辛酸泪