javascript - What does the `undefined` mean in node.js console? -

i wrote code in node.js console, , noticed return undefined.
see following code.

some lines return undefined don't.
undefined mean?

ps d:\test\hello> node > "two\nlines" 'two\nlines' > var s = "hello, world"      undefined > s.charat(0) 'h' > s.charat(s.length -1) 'd' > var anystring = 'mozilla'; undefined > console.log(anystring.substring(3, 0)); moz undefined 

but last line , returns undefined returns moz?

> console.log(anystring.substring(3, 0)); moz undefined 

console.log explicitly outputs console, moz you're seeing. returns value (or well, doesn't), value undefined, logged console because you're in interactive environment , that's does.

the point of using interactive command line able try operations , see result instantly, without complex alert or debugging setups. type command, see result. result value undefined, it's value.