javascript - Using Async waterfall in node.js

Question

I have 2 functions that I'm running asynchronously. I'd like to write them using waterfall model. The thing is, I don't know how..

Here is my code :

var fs = require('fs');
function updateJson(ticker, value) {
  //var stocksJson = JSON.parse(fs.readFileSync("stocktest.json"));
  fs.readFile('stocktest.json', function(error, file) {
    var stocksJson =  JSON.parse(file);

    if (stocksJson[ticker]!=null) {
      console.log(ticker+" price : " + stocksJson[ticker].price);
      console.log("changing the value...")
      stocksJson[ticker].price =  value;

      console.log("Price after the change has been made -- " + stocksJson[ticker].price);
      console.log("printing the the Json.stringify")
      console.log(JSON.stringify(stocksJson, null, 4));
      fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function(err) {  
        if(!err) {
          console.log("File successfully written");
        }
        if (err) {
          console.error(err);
        }
      }); //end of writeFile
    } else {
      console.log(ticker + " doesn't exist on the json");
    }
  });
} // end of updateJson 

Any idea how can I write it using waterfall, so i'll be able to control this? Please write me some examples because I'm new to node.js

Answer 1

First identify the steps and write them as asynchronous functions (taking a callback argument)

• read the file

  function readFile(readFileCallback) {
      fs.readFile('stocktest.json', function (error, file) {
          if (error) {
              readFileCallback(error);
          } else {
              readFileCallback(null, file);
          }
      });
  }
  

• process the file (I removed most of the console.log in the examples)

  function processFile(file, processFileCallback) {
      var stocksJson = JSON.parse(file);
      if (stocksJson[ticker] != null) {
          stocksJson[ticker].price = value;
          fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function (error) {
              if (err) {
                  processFileCallback(error);
              } else {
                  console.log("File successfully written");
                  processFileCallback(null);
              }
          });
      }
      else {
          console.log(ticker + " doesn't exist on the json");
          processFileCallback(null); //callback should always be called once (and only one time)
      }
  }
  

Note that I did no specific error handling here, I'll take benefit of async.waterfall to centralize error handling at the same place.

Also be careful that if you have (if/else/switch/...) branches in an asynchronous function, it always call the callback one (and only one) time.

Plug everything with async.waterfall

async.waterfall([
    readFile,
    processFile
], function (error) {
    if (error) {
        //handle readFile error or processFile error here
    }
});

Clean example

The previous code was excessively verbose to make the explanations clearer. Here is a full cleaned example:

async.waterfall([
    function readFile(readFileCallback) {
        fs.readFile('stocktest.json', readFileCallback);
    },
    function processFile(file, processFileCallback) {
        var stocksJson = JSON.parse(file);
        if (stocksJson[ticker] != null) {
            stocksJson[ticker].price = value;
            fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function (error) {
                if (!err) {
                    console.log("File successfully written");
                }
                processFileCallback(err);
            });
        }
        else {
            console.log(ticker + " doesn't exist on the json");
            processFileCallback(null);
        }
    }
], function (error) {
    if (error) {
        //handle readFile error or processFile error here
    }
});

I left the function names because it helps readability and helps debugging with tools like chrome debugger.

If you use underscore (on npm), you can also replace the first function with _.partial(fs.readFile, 'stocktest.json')