CODEWITHSUNDEEP
javascript
Wei Ma
Wei Ma

Reputation: 3155

Java script modify function at run time

enter code hereI have the following code

function a(){alert("a");}

I want to create a function b as

function b(){alert("a"); alert("b");}

My approach is something like 

var b = a + alert("b");

This is of course not working. But I am wondering if there is some kind of library supporting this.

Edit: Maybe I need to describe my scenario so that its more clear what I want to achieve.

I am using async.js library to handler multiple async calls. My code looks like

var values = {};
...
function all() {
    var allDfd = $.Deferred();
    async.parallel(
            [function (callback) {
                remoteCall(function (result) {
                    values.v1 = result;
                    callback(null, 'one');
                });
            },
            function (callback) {
                remoteCall(function (result) {
                    values.v2 = result;
                    callback(null, "two");
                });
            },
            function (callback) {

                remoteCall(function (result) {

                    values.v3 = result;
                    callback(null, "three");
                });
            }], function (err, results) {
                allDfd.resolve();
            });
    return allDfd.promise();
}

Clearly there are a lot of repetitive code that bothers me. So my idea is to create a function asyncCall to perform the boilerplate tasks. The idea is 

   var values = {};
...
function all() {
    var allDfd = $.Deferred();
    function getAsyncCall (func, innerCallback, callback) {
        return function asyncCall(func, innnerCallback, callback){
            func(innerCallback + callback(null));  // combine innerCallBack and callback code
        }
    }

    async.parallel(
            [getAsyncCall(remoteCall, function(result){values.v1=result;},callback),
             getAsyncCall(remoteCall, function(result){values.v2=result;},callback),
             getAsyncCall(remoteCall, function(result){values.v3=result;},callback),
            ], function (err, results) {
                allDfd.resolve();
            });
    return allDfd.promise();
}

The line with the comment is what I am pondering. I am trying to create a new function that combines inner and outer callbacks.

Upvotes: 0

Views: 430

Answers (2)

xtofl
xtofl

Reputation: 41509

You could write:

var a=function(){alert("a");}

var b=function(){a(); alert("b");}

And to go a little further, you can even write a whole function composition function:

function compose( functions ) {
   return function(){ 
      for(var i=0; i!=functions.length; ++i) {
        functions[i]();
      }
   };
}

var c=compose( [a, function(){ alert("b"); }] );

(See it at work at http://jsfiddle.net/xtofl/Pdrge/)

Upvotes: 1

Jaime
Jaime

Reputation: 6814

You can do 

var b = function() { a(); alert('b'); }

Upvotes: 4

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