nodejs-函数返回未退出

Question

I have a function that does a series of tests. If the first test fails the function should exit, however it doesn't:

function checkSite(site) {
console.log("Checking if site is already processed or processing");
// Processed Sites
fs.readFile("processed.txt", "utf8", function(error, data) {
 if (data.indexOf(site) > -1) {
  console.log("Processed");
  return "Processed";
 } else {
  console.log("Not Processed");
 }
});
fs.readFile("processing.txt", "utf8", function(error, data) {
if (data.indexOf(site) > -1) {
 console.log("Processing");
 return "Processing";
} else {
 console.log("Not Processing, Queue for Processing");
 // Queue for Processing
 fs.appendFile(fileProcessing, site, function (err) {
  if (err) throw err;
  console.log('The "data to append" was appended to file!');
 });
}
});
return "Success";
}

I'm getting console output of "Processed" followed by "Processing". I should never see "Processing" with the return there. What am I missing here?

Answer 1

return "Processed" is just returning from the callback function, not from checkSite() so it does not influence what happens next.

fs.readFile() is asynchronous. If you want the second fs.readFile() to wait for the first to complete or you want to not execute further actions after the first fs.readFile() finishes successfully, you will have to code this differently.

Here's one way to do it:

function checkSite(site) {
    console.log("Checking if site is already processed or processing");
    // Processed Sites
    fs.readFile("processed.txt", "utf8", function(error, data) {
        if (data.indexOf(site) > -1) {
            console.log("Processed");
        } else {
            console.log("Not Processed");
            fs.readFile("processing.txt", "utf8", function(error, data) {
                if (data.indexOf(site) > -1) {
                    console.log("Processing");
                } else {
                    console.log("Not Processing, Queue for Processing");
                    // Queue for Processing
                    fs.appendFile(fileProcessing, site, function(err) {
                        if (err) throw err;
                        console.log('The "data to append" was appended to file!');
                    });
                }
            });
        }
    });
}

Note: you can't return the success or failure result directly from checkSite() because the fs.readFile() operations are asynchronous and thus checkSite() returns before those operations are done. If you want to communicate the result back to the caller of checkSite() then, you will either need to pass in a callback function that gets called with the result, use the structure of promises to help you with that or switch to synchronous functions instead of async versions (not usually recommended).