CODEWITHSUNDEEP
javascriptnode.jslexical-scope
rohit12sh
rohit12sh

Reputation: 847

Nodejs asyn.apply and Javascript Apply

Can someone please explain me why the below code prints "arg1 0" while I expect it to print "arg1 hi". Is it due to the lexical scope?

runIt();

function localScoped(arg1, callback) {
  console.log('arg1', arg1);
  callback();
}

function runIt() {

   var myValue = 0;
   async.eachLimit(["hi"], 1,
      function launchOneVar(clientCode, doneLaunchOneVar) {

          async.waterfall([
              function (myCallback) {
                  myValue = clientCode;
                  myCallback();
              },
              async.apply(localScoped, myValue)
          ], function (err, result) {
              console.log(myValue);
              doneLaunchOneVar(null, result);
          });
      },
     function finishing(err) {
     }
  );

}

Upvotes: 1

Views: 215

Answers (2)

vp_arth
vp_arth

Reputation: 14992

Waterfall is just a function.
There is not any magic here.
Any function in Javascript must evaluate its arguments before call.

So, async.apply(localScoped, myValue) evaluates before async.waterfall and closured old myValue's value (0);

You can pass it through waterfall:

function (myCallback) {
  myValue = clientCode;
  myCallback(null , myValue);
},
async.apply(localScoped);

Or write a wrapper function, like:

function(next) {
  localScoped(myValue, next);
}

Upvotes: 1

Chetan Ameta
Chetan Ameta

Reputation: 7896

try below solution:

runIt();

function localScoped(arg1, callback) {
  console.log('arg1', arg1);
  callback();
}

function runIt() {
   var myValue = 0;
   async.eachLimit(["hi"], 1,
      function launchOneVar(clientCode, doneLaunchOneVar) {
        //console.log(clientCode);
          async.waterfall([

              function (myCallback) {
                  myValue = clientCode;
                  myCallback(null , myValue);
              },
              async.apply(localScoped)

          ], function (err, result) {
              console.log('last - '+myValue);
              doneLaunchOneVar(null, result);
          });
      },
     function finishing(err) {
     }
  );
}

how water fall works: .

Runs a list of async tasks, passing the results of each into the next one.

Runs an array of functions in series, each passing their results to the next in the array. However, if any of the functions pass an error to the callback, the next function is not executed and the main callback is immediately called with the error.

For more detail have a look at: https://www.npmjs.com/package/async-waterfall

Upvotes: 0

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