JS callback using map() with a function that has one additional parameter [duplicate]

Question

I'm trying to find a way to use JS's Array.prototype.map() functionality with a function that has one additional parameter more (if possible at all, and I'd like to avoid having to rewrite built-in Array.prototype.map()). This documentation is very good, but does not cover the "one-or-more-additional-parameter" case:

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/map

function doOpSingle(elem)
{
 // do something with one array element
}

var A = ["one", "two", "three", "four"];
var x = A.map(doOpSingle); // this will execute doOpSingle() on each array element

So far, so good. But what if the function in question has two parameters, like e. g. a flag you might want to OR to it (think of bit masks)

function doOpSingle2(arrelem,flag)
{
 // do something with one array element
}

var A = ["one", "two", "three", "four"];
var theFlag = util.getMask(); // call external function
var y = A.map(doOpSingle2(theFlag)); // this does not work!

Any solutions should do without for loops, of course, because that's why we have map(), to make our code cleaner w/getting rid of these!

NorthDecoder · Accepted Answer · 2017-03-04 12:03:12Z

20

Use an anonymous function:

A.map(function(a) {return doOpSingle2(a,theFlag);});

edited Mar 4, 2017 at 12:03

NorthDecoder

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answered Jun 3, 2012 at 17:22

Niet the Dark Absol

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2 Comments

UltraInstinct Over a year ago

+1, realised after posting my answer :)

syntaxerror Over a year ago

TY! This will most likely be the only way to cover this task (apart from bind()). Thanks to the other answerers as well.

Esailija · Accepted Answer · 2012-06-03 17:27:18Z

14

A.map(doOpSingle2.bind(null, theFlag))

theFlag will be first argument though:

function doOpSingle2( flag, elem ) { ... }

answered Jun 3, 2012 at 17:27

Esailija

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1 Comment

syntaxerror Over a year ago

Love this bind() technique, thanks!

UltraInstinct · Accepted Answer · 2012-06-03 17:23:05Z

1

Use an anonymous function.

var y = A.map(function(x){doOpSingle2(x,theFlag);});

answered Jun 3, 2012 at 17:23

UltraInstinct

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Comments

Tyler · Accepted Answer · 2012-06-03 17:39:12Z

1

Edit: Nevermind, you don't really need Closure for this. See Esailija's answer.

There is another option, if you happen to be using the Closure library. First you have to rewrite doOpSingle2 so that the flag is first:

function doOpSingle2(flag, arrelem)
{
 // do something with one array element
}

then you can do

var y = A.map(goog.partial(doOpSingle2, theFlag));

goog.partial partially applies the function, so goog.partial(doOpSingle2, theFlag) is equivalent to this function:

function(arrElem) {
  return doOpSingle2(theFlag, arrElem);
}

In this particular case, I probably wouldn't recommend this approach, but there might be similar situations where it works well.

edited Jun 3, 2012 at 17:39

answered Jun 3, 2012 at 17:29

Tyler

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3 Comments

Esailija Over a year ago

javascript can already do currying with .bind though

syntaxerror Over a year ago

THIS is the very problem. It would just be overkill to use an external library for this little problem. But still, thanks anyway for this approach. In a real big project, I might need this technique someday. ;)

Tyler Over a year ago

That's why I said "if you happen to be using Closure" instead of "you should drop everything right now and start using Closure" the way some people do ;).

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