It's a reducer, actually

gistfile1.txt

// "pure" version
[1, 2, 3, 4].reduce(function(acc, elem) {
  var result = [];
  // Generate multiple "elements".
  for (var i = 0; i < elem; i++) {
    result.push(i);
  }
  // And generate the new "array" as the new accumulator.
  return acc.concat(result);
}, [])

// "impure" version
[1, 2, 3, 4].reduce(function(acc, elem) {
  var result = [];
  // Generate multiple "elements".
  for (var i = 0; i < elem; i++) {
    acc.push(i);
  }
  // remember to return it!
  return acc;
}, [])

// lo-dash named it as: _.flatten:
// https://lodash.com/docs#flatten

var theNestedArray = (function(arr){
  return arr.map(function(elem) {
    var result = [];
    for (var i = 0; i < elem; i++) {
      result.push(i);
    }
    return result;
  });
})([1,2,3,4])

_.flatten(theNestedArray, true);