Angular: infinite digest loop in filter

Question

I'm writing a custom Angular filter that randomly capitalizes the input passed to it.

Here's the code:

angular.module('textFilters', []).filter('goBananas', function() {
  return function(input) {

    var str = input;
    var strlen = str.length;

    while(strlen--) if(Math.round(Math.random())) {
      str = str.substr(0,strlen) + str.charAt(strlen).toUpperCase() + str.substr(strlen+1);
    }

    return str;
  };
});

I call it in my view like so:

    <a class='menu_button_news menu_button' ng-href='#/news'>
        {{"News" | goBananas}}
    </a>

It works, but in my console I'm seeing a rootScope:infdig (infinite digest) loop.

I'm having some trouble understanding why this is happening and what I can do to resolve it. If I understand correctly, this is due to the fact that there are more than 5 digest actions called by this function. But the input is only called once by the filter, right?

Any help appreciated.

Answer 1

The problem is that the filter will produce a new result every time it is called, and Angular will call it more than once to ensure that the value is done changing, which it never is. For example, if you use the uppercase filter on the word 'stuff' then the result is 'STUFF'. When Angular calls the filter again, the result is 'STUFF' again, so the digest cycle can end. Contrast that with a filter that returns Math.random(), for example.

The technical solution is to apply the transformation in the controller rather than in the view. However, I do prefer to transform data in the view with filters, even if the filter applies an unstable transformation (returns differently each time) like yours.

In most cases, an unstable filter can be fixed by memoizing the filter function. Underscore and lodash have a memoize function included. You would just wrap that around the filter function like this:

.filter('myFilter', function() {
  return _memoize(function(input) {
    // your filter logic
    return result;
  });
});

Answer 2

Since digest will continue to run until consistent state of the model will be reached or 10 iterations will run, you need your own algorithm to generate pseudo-random numbers that will return the same numbers for the same strings in order to avoid infinite digest loop. It will be good if algorithm will use character value, character position and some configurable seed to generate numbers. Avoid using date/time parameters in such algorithm. Here is one of possible solutions:

HTML

<h1>{{ 'Hello Plunker!' | goBananas:17 }}</h1> 

JavaScript

angular.module('textFilters', []).
  filter('goBananas', function() {
    return function(input, seed) {
      seed = seed || 1;
      (input = input.split('')).forEach(function(c, i, arr) {
        arr[i] = c[(c.charCodeAt(0) + i + Math.round(seed / 3)) % 2 ? 'toUpperCase' : 'toLowerCase']();
      });
      return input.join('');
    }
  });

You can play with seed parameter to change a bit an algorithm. For example it may be $index of ngRepeat

Plunker: http://plnkr.co/edit/oBSGQjVZjhaIMWNrPXRh?p=preview