I want to call a function with an argument and store the value so that next time I call that function without arguments it uses the last argument that was set. Is this possible with JavaScript?

Edit: Here's more info on what I'm trying to achieve...

var track = 0;
$.getJSON('songsMetadata.json', function(data){
    appendData(data);
});
player.bind("ended", function(){
    track++
    appendData();
});
function appendData(data){
    /* Here I want to populate 
     the inside 'data' argument 
    only once (in the callback of 
    getJSON) and use that same
    data in the 'player.bind' 
    callback */

    //Some code to append data[track] to an HTML list
}

I want to call a function with an argument and store the value so that next time I call that function without arguments it uses the last argument that was set. Is this possible with JavaScript?

Edit: Here's more info on what I'm trying to achieve...

var track = 0;
$.getJSON('songsMetadata.json', function(data){
    appendData(data);
});
player.bind("ended", function(){
    track++
    appendData();
});
function appendData(data){
    /* Here I want to populate 
     the inside 'data' argument 
    only once (in the callback of 
    getJSON) and use that same
    data in the 'player.bind' 
    callback */

    //Some code to append data[track] to an HTML list
}

• javascript
• function
• persistence

Share Improve this question Follow edited Jun 10, 2017 at 21:40 medicengonzo asked Jun 10, 2017 at 19:47 medicengonzomedicengonzo 51922 gold badges1010 silver badges2323 bronze badges 5

• yeah, sure it can be done but if you could please give me a bit more information, i could write you an example. what will the argument be? – Molik Miah Commented Jun 10, 2017 at 19:48
• Possible duplicate of Javascript: Creating a function with state – Andreas Commented Jun 10, 2017 at 20:02
• You may be interested in iterators and generators – BoltKey Commented Jun 10, 2017 at 20:06
• @MolikMiah the argument will be an array that I get from an Ajax call to a JSON file. The array contains mp3s metadata from an HTML 5 playlist that I coded. What I want to do is append this data gradually into the HTML (showing the metadata as the playlist plays each song). I'm still a little bit confused about asynchronous code – medicengonzo Commented Jun 10, 2017 at 20:41
• You can use either of the existing answers to acplish this. Based on my answer for example - you just have to wrap appendData in the wrapper function. var cachedAppend = cacheFn(appendData); then use cacheAppend in all places you would use appendData. This normalizes appendData so it always gets called with some argument, either the one being passed, or the last argument to have been passed (as you specified in your question). – Damon Commented Jun 10, 2017 at 23:24

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2 Answers 2

Sorted by: Reset to default 4

You need to keep a reference to the last acceptable argument in the enclosing scope. For example:

var ref;

function foo (arg) {
  if (!arg) { // or arg === undefined if you want to be safe
    arg = ref;
  } else {
    ref = arg;
  }

  // actual function behavior here
  console.log(arg);
}

foo(); // undefined
foo(2); // 2
foo(); // 2
foo(3); // 3
foo(); // 3

If you want to repeat this behavior, you might want to consider writing a wrapper function to cache an accepted function's argument. For example:

function cacheFn (fn) {
  var ref;
  
  return function (arg) {
    if (!arg) {
      arg = ref;
    } else {
      ref = arg;
    }
    
    return fn(arg);
  }
}

function foo (arg) {
  console.log(arg);
}

var cachedFoo = cacheFn(foo);

cachedFoo(2);
cachedFoo();
cachedFoo(3);
cachedFoo();

In a more general way:

function enclose(func) {
 let args = [];
 return function (...passed) {
   if(passed.length) args = passed;
   func(...args);
 };
}

Usecase:

const log = enclose(console.log.bind(console));
log(5,1);
log();