Node.js：如何防止两个回调同时运行？

Question

I'm a bit new to Node.js. I've run into a problem where I want to prevent a callback from running while it is already being executed. For example:

items.forEach(function(item) {
    doLongTask(item, function handler(result) {
        // If items.length > 1, this will get executed multiple times.
    });
});

How do I make the other invocations of handler wait for the first one to finish before going ahead? I'm thinking something along the lines of a queue, but I'm a newbie to Node.js so I'm not exactly sure what to do. Ideas?

Answer 1

How to prevent two callbacks from running simultaneously?

They won't run simultaneously unless they're asynchronous, because Node runs JavaScript on a single thread. Asynchronous operations can overlap, but the JavaScript thread will only ever be doing one thing at a time.

So presumably doLongTask is asynchronous. You can't use forEach for what you'd like to do, but it's still not hard: You just keep track of where you are in the list, and wait to start processing the next until the previous one completes:

var n = 0;
processItem();
function processItem() {
    if (n < items.length) {
        doLongTask(items[n], function handler(result) {
            ++n;
            processItem();
        });
    }
}

Answer 2

There are already libraries which take care of that, the most used being async .

You will be interested in the async.eachSeries() function.

As for an actual example...

const async = require('async')

async.eachSeries(
  items,
  (item, next) => {
    // Do stuff with item, and when you are done, call next
    // ...
    next()
  },
  err => {
    // either there was an error in one of the handlers and
    // execution was stopped, or all items have been processed
  }
)

As for how the library does this, you are better of having a look at the source code.

It should be noted that this only ever makes sense if your item handler ever performs an asynchronous operation, like interfacing with the filesystem or with internet etc. There exists no operation in Node.js that would cause a piece of JS code to be executed in parallel to another JS code within the same process. So, if all you do is some calculations, you don't need to worry about this at all.