bend a map function to do other things

utils-from-map.js

// where map(x, function(y, i, z){ z === x; return y }) === x
// and i is the index/key, and z is the first argument to map
// e.g. utilsFromMap(function(array, fn){ array.map(fn) });
// e.g. utilsFromMap(React.Children.map);
function utilsFromMap(map){
  var utils = {};
  
  // herein fn is a function, 
  // X is the thing to be passed to map as the first argument
  // and there other parameters are specific to the function
  
  // utils.map = flip(map);
  // js is weird
  utils.map = function(fn, x){ return map(x, fn); };
  
  
  utils.reduce = function(fn, start, X){
    var acc = start;
    map(X, function(x){
      acc = fn(acc, x);
      return null;
    });
    return acc;
  };
  
  utils.toArray = function(X){
    var items = [];
    map(X, function(x){ items.push(x); });
    return items;
  };
  
  utils.reverse = function(X){
    var items = utils.toArray(X);
    var counter = items.length;
    return map(X, function(x, i){ 
      counter--;
      return items[counter] 
    });
  };
  
  utils.filter = function(fn, X){
    return map(X, function(x, i, X){
      return fn(x, i, X) ? x : null;
    });
  };
  
  utils.length = utils.reduce.bind(null, 0, function(acc){
    return acc+1;
  });
  
  return utils;
}