Info

This is an Array#filter implementation from a recent 140bytes code golf entry. It's faster than native Array#filter when iterating over large sparse arrays because it doesn't use the length property to calculate when it should stop iterating.

Preparation code

<script>
Array.prototype.filter2 = function(
  a, //a function to test each value of the array against. Truthy values will be put into the new array and falsey values will be excluded from the new array
  b, // placeholder
  c, // placeholder 
  d, // placeholder
  e // placeholder
) {
  c = this; // cache the array
  d = []; // array to hold the new values which match the expression
  for (e in c) // for each value in the array, 
    ~~e + '' == e && e >= 0 && // coerce the array position and if valid,
    a.call(b, c[e], +e, c) && // pass the current value into the expression and if truthy,
    d.push(c[e]); // add it to the new array
  
  return d // give back the new array
};

Array.prototype.filter3 = function(truthtest) {
        var results = [];
        for (var index=0, length=this.length; index<length; index++) {
                if (truthtest.call(this[index], index, this))
                        results.push(this[index])
        }
        
        return results;
}

Array.prototype.filter4 = function(truthtest) {
        var results = [];
        for (var key in this) {
                var numericKey = key+0;
                if (numericKey == numericKey && truthtest.call(this[numericKey], numericKey, this))
                        results.push(this[numericKey])
        }
        
        return results;
}
</script>
<script>
  Benchmark.prototype.setup = function() {
    var sparse_array = []
    sparse_array[200] = 'a'
    
    var dense_array = [];
    for(var i=0; i<200; i++) {
      dense_array[i] = String.fromCharCode(Math.round(Math.random() * 95 + 33))
    }
    
    
    function callback(v) {
      return /\w+/.test(v);
    }
  };
</script>

Test runner

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Java applet disabled.

sparse_array.filter(callback);

sparse_array.filter2(callback);

dense_array.filter(callback);

dense_array.filter2(callback);

sparse_array.filter3(callback);

dense_array.filter3(callback);

sparse_array.filter4(callback);

dense_array.filter4(callback);

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Revisions

You can edit these tests or add even more tests to this page by appending /edit to the URL. Here’s a list of current revisions for this page:

• Revision 1: published by John-David Dalton on 17th October 2011
• Revision 2: published on 18th October 2011
• Revision 3: published on 18th October 2011
• Revision 4: published on 13th December 2012
• Revision 5: published on 15th January 2013

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