Node.js Tutorial - Node.js Asynchronous Programming

The following code shows how Node.js handls the nonblocking, asynchronous model.

setTimeout() function takes a function to call and a timeout after which it should be called:


setTimeout(function () {
    console.log("done");
}, 2000);

console.log("waiting");

If you run the preceding code, you see the following output:

The program sets the timeout for 2000ms (2s), and then continues with execution, which prints out the "waiting" text.

In Node.js, to call a function that needs to wait for some external resource, instead of calling fopen(path, mode) and waiting, we should call fopen(path, mode, function callback(file_handle) { ... }).

Example

The following code asynchronous functions.


var fs = require('fs');
/*  w ww .ja v a2s.c  om*/
fs.open(
     'a.js', 'r',
      function  (err, handle) {
         var buf = new Buffer(100000);
         fs.read(
             handle, buf, 0, 100000, null,
              function  (err, length) {
                 console.log(buf.toString('utf8', 0, length));
                 fs.close(handle, function () { /* don't care */ });
             }
         );
     }
);

The code above generates the following result.

Note

The require function is a way to include additional functionality in your Node.js programs.

callback asynchronous functions has at least one parameter, the success or failure status of the last operation. It commonly has a second parameter which has additional results or information from the last operation.


do_something(param1, param2, ..., paramN,  function (err, results)  { ... });

For example,


fs.open(//  w w  w . j a  v  a 2 s.  c o  m
     'a.js', 'r',
     function (err, handle) {
          if (err) {
              console.log("ERROR: " + err.code + " (" + err.message ")");
              return;
          }
         // success!! continue working here
     }
);

In this style, you check for errors, or continue to process the result. And now here's the other way:


fs.open(//  w  w  w. j  a va 2 s.  co  m
     'a.hs', 'r',
     function (err, handle) {
          if (err) {
              console.log("ERROR: " + err.code + " (" + err.message ")");
          } else {
             // success! continue working here
          }
      }
);

The following code shows how to read a file with error handler.


var fs = require('fs');
//  w w  w. j a v a  2s .  c o  m
fs.open(
    'a.js', 'r',
     function  (err, handle) {
        if (err) {
            console.log("ERROR: " + err.code + " (" + err.message + ")");
            return;
        }
        var buf = new Buffer(100000);
        fs.read(
            handle, buf, 0, 100000, null, function  (err, length) {
                if (err) {
                    console.log("ERROR: " + err.code + " (" + err.message + ")");
                    return;
                }
                console.log(buf.toString('utf8', 0, length));
                fs.close(handle, function () { /* don't care */ });
            }
        );
    }
);

The code above generates the following result.