why js控制台console总是会输出undefined？

Question

RT

js控制台console.log输出，为什么一直会有undefined结果？

Answer 1

之前回答过同样的问题，题主可参考：

http://segmentfault.com/q/1010000003821273/a-1020000003821281

简言之控制台会返回给你上一段代码的最后返回的执行结果

function a() {return 2;}
var b = 3
>> undefined
a()
>> 2
a();b;
>> 3

Answer 2

If you're running console.log() from a JS file, this undefined line should not be appended.

If you're running console.log() from the console itself, it makes sense. This is why: In the console you can type a name of a variable (for example try typing window) and it prints info about it. When you run any void function (like console.log) from the console, it also prints out info about the return value, undefined in this case.

I tested both cases on my Chrome (Mac ver 23.0.1271.101) and indeed I see the undefined line when I run it inside the console. This undefined also appears when I write this line in the console: var bla = "sdfdfs"

answered Jan 31 '13 at 19:57