node.js 在控制台上显示“未定义”

Question

Recently I installed node.js on my Windows 7 machine.

On execution of JavaScript, I get an undefined message along with successful execution of the expression.

What's wrong here? I have not noticed any other side effects.

Answer 1

The JavaScript functions always return something. If you don't specify something to return in the function, 'undefined' is returned by default (you can check this out in Firebug too).

Don't worry though, this doesn't affect anything, you can ignore it.

Answer 2

Just write "hello world"; and hit enter... it will return "hello world" instead of undefined , thus no undefined is displayed. console.log returns undefined and also logs arguments to console so you get multiple messages.

Answer 3

As pointed out by others, javascript function will always return undefined if you do not specify any return value. You can just ignore it. It's not going to cause any harm. But if it's annoying you too much then you can turn it off in repl. Repl has this property ignoreUndefined which is set to false by default. You can set it to true. Try this:

module.exports.repl.ignoreUndefined = true;

Answer 4

Well, the question was made some years ago, but there is still another way to explain what is happening here.

Try next command:

console.log("Hello World") || "Bye World";

As mentioned function console.log() returns undefined normally and you can choose a better return value. Since Non-strict (abstract) comparison considers undefined equal to false the || operator allows you that choice.

Considering that it is just a debugging tool, this is unnecessary in general use, but helps to understand that console displays text sent to stdout and also evals the command and displays the returned value, or undefined if no value was received.

Answer 5

如果您需要从模块内部打开ignoreUndefined ，您可以使用以下行来模拟AvinashB的答案，而无需自己在控制台中输入：

process.stdin.emit('data', "module.exports.repl.ignoreUndefined = true;\n");

Answer 6

node也会打印undefined因为IN GENERAL ，它显示每个命令的返回值，并且console.log不返回任何内容。

Answer 7

this is happening because console.log has (null/undefined) return type to solve this issue.. have a function returning a string.. then log that function returned value... that will overcome the undefined issue..

const addNotes= function(msg)
{
    return msg;
}
console.log(addNotes('say hello'));

Answer 8

Just ignore it, no need to worry at all. I too had the same issue when I called the function without any return type. So node used to print undefined return type for the called function.

function emitEvent(eventname){
    eventEmitter.emit(SIV_EVENT);
}
console.log(emitEvent());

Check undefined here, https://nodejs.org/api/util.html .

Answer 9

为了避免看到undefined使用节点的时候，从命令的输出repl模块，设置ignoreUndefined到true的repl.start 。