Python list comprehensions nodejs/javascript

Question

Is there anything similar to pythons list comprehension for nodejs/javascript? If there is not then is it possible to make a function that has similar behavior for example

# Example 1

list_one = [[1, 2], [3, 4], [5, 6], [7, 8]]
someOfList = sum(x[1] for x in list_one)
print(someOfList) # prints 20

# Example 2
combined = "".join([str( ( int(x) + int(y) ) % 10) for x, y in zip("9999", "3333")])
print(combined) # prints 2222

Etc? Or would you have to make functions for each comprehension like behavior? I know you can make functions for each of those, but if you use a lot of list comprehensions code can get long

This question was asked a few months ago [here](http://stackoverflow.com/questions/14511954/array-comprehensions-in-nodejs) but as things move pretty fast, I guess this question is not a duplicate. — Ray Toal, Jan 27 '14 at 03:27

score 14 · Accepted Answer · answered Jan 27 '14 at 03:41

14

List comprehensions put into a language's syntax what would normally be done with map and filter.

So given a Python list comprehension, you can also use map and filter:

# Python - preferred way
squares_of_odds = [x * x for x in a if x % 2 == 1]

# Python - alternate way
map(lambda x: x * x, filter(lambda x: x % 2 == 1, a))

although comprehensions are preferred in Python. JavaScript has map and filter so you can use those now.

// JavaScript
a.map(function(x){return x*x}).filter(function(x){return x%2 == 1})

The upcoming version of JavaScript will have array comprehensions in the language:

[ x*x for (x of a) if (x % 2 === 1) ]

To see what is available now in the upcoming version, see this compatibility table. As of this writing, you can see they are available in Firefox.

answered Jan 27 '14 at 03:41

Ray Toal

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Can you test this array comprehension in NodeJS? I tried in Node but `var myArray = ["one", 2, 3.0, [4, 5, "six"]]; [x for (x of myArray)]` SyntaxError: Unexpected token for. I could not get it working in Chrome console as well. – hhh Sep 19 '18 at 20:29
1

Oh sorry this is a 4-1/2 year old answer, and at the time I wrote it, it was looking like JS was going to get array comprehensions, but that did not happen. It is currently a non-standard feature: http://kangax.github.io/compat-table/non-standard/ that is not supported in Node, nor anywhere else really! Things are fluid, it would seem. :) – Ray Toal Sep 19 '18 at 22:48

score 0 · Answer 2 · answered Jan 27 '14 at 03:31

this is why coffee script was invented, you can give in a try by copying and pasting your code here (go to "try cofeescript" tab)

this is what it just gave me:

var combined, list_one, someOfList, x, y;

list_one = [[1, 2], [3, 4], [5, 6], [7, 8]];

someOfList = sum((function() {
  var _i, _len, _results;
  _results = [];
  for (_i = 0, _len = list_one.length; _i < _len; _i++) {
    x = list_one[_i];
    _results.push(x[1]);
  }
  return _results;
})());

print(someOfList);

combined = "".join([
  (function() {
    var _i, _len, _ref, _results;
    _ref = zip("9999", "3333");
    _results = [];
    for (y = _i = 0, _len = _ref.length; _i < _len; y = ++_i) {
      x = _ref[y];
      _results.push(str((int(x) + int(y)) % 10));
    }
    return _results;
  })()
]);

Python list comprehensions nodejs/javascript

2 Answers2