Utilizing array.prototype.forEach() correctly

Question

I would love to utilize a function that strips out items from something like .forEach(). An example of what I'm using currently looks like:

// return items even if no authentication is present
return ArticleModel.find(function (err, articles) {
  var regularArray = [];
  function stripOutPremium(element, index, array) {
  if (element.is_premium === true) {
    var elementDescAdjustment = element.description;
    element.description = 'Premium content!  ' + elementDescAdjustment.substr(0,15) + '...';
    regularArray.push(element);
  } else {
    regularArray.push(element);
  }
}
articles.forEach(stripOutPremium);
return res.send(regularArray);

However, this function stripOutPremium() is going to be used in a lot of different places, for this particular application (shown is a bit of a single Express App Route). How can I rewrite this function outside of this return statement so that I can use it on any route in my app? I know it involves doing something new with the var regularArray = []; and possibly even the prototype aspect of JS to do this.

@user2864740 The function adds all of the stuff into the array, it just changes some of them. — Scimonster
– Scimonster, Commented Nov 13, 2014 at 21:00
To elaborate on what @Scimonster said, why are you defining a new array at all? The new array has the exact same objects in the exact same order as the original. You've simply performed a mutation on some of the objects while making the new array -- why not perform the mutations and keep the same array? If you plan to make inclusion in the new array conditional in future development, then your current approach makes sense, though. — apsillers
– apsillers, Commented Nov 13, 2014 at 21:02
From my little understanding of Array.map - it also returns a new array. So how can I mutate an array without returning a new on, while at the same time breaking out elements of the array and mutating them individually? — user393219
– user393219, Commented Nov 13, 2014 at 21:24

Scimonster · Accepted Answer · 2014-11-13 21:08:13Z

2

I think your best bet is to use Array.map(), and return a new version of the array:

articles.map(function(element, index, array) {
  if (element.is_premium === true) {
    var elementDescAdjustment = element.description;
    element.description = 'Premium content!  ' + elementDescAdjustment.substr(0,15) + '...';
  }
  return element;
});

answered Nov 13, 2014 at 21:08

Scimonster

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Comments

apsillers · Accepted Answer · 2014-11-13 21:13:11Z

First, as mentioned in the comments, use filter instead of forEach here:

function stripOutPremium(element, index, array) {
    if (element.is_premium === true) {
        var elementDescAdjustment = element.description;
        element.description = 'Premium content!  ' + elementDescAdjustment.substr(0,15) + '...';
    }
    return true;
}
return articles.filter(stripOutPremium);

filter builds a new array, so you don't need to make one yourself. By return true in the callback, you include every element of the original array, but you can conditionally mutate each element as you process it. If you need to exclude any elements in the future, you can return false for that element instead. (You can also do the same thing with map, but do return element instead of return true.)

To answer your actual question: if you want a use a general function as a callback but have it mutate some particular local array, you can write a function that returns another function. The outer function takes the array as an argument and returns a newly-created inner function that uses the array:

function generateStriptOutPremiumForArray(someArray) {
    return function stripOutPremium(element, index, array) {
      if (element.is_premium === true) {
        var elementDescAdjustment = element.description;
        element.description = 'Premium content!  ' + elementDescAdjustment.substr(0,15) + '...';
        someArray.push(element);
      } else {
        someArray.push(element);
      }
    }
}

Here, the outer function returns a new function that has the outer function's someArray variable in its scope chain. Since you passed in regularArray as the outer function's someArray argument, the generated inner function has access to the local regularArray.

Call it with:

var regularArray = [];
var generatedCallbackForThisRegularArray = generateStripOutPremiumForArray(regularArray);
articles.forEach(generatedCallbackForThisRegularArray);

// or just: articles.forEach(generateStripOutPremium(regularArray));

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