counting things in javascript

count.js

var count = function (sequence) {
  return sequence.reduce(function (tally, item) {
    if (!(item in tally)) {
      tally[item] = 0
    }
    tally[item] += 1;
    return tally
  }, {
  })
}

var words = 'pease porridge hot pease porridge cold pease porridge in the pot nine days old'.split(' ');

console.log(count(words))

//{ pease: 3, porridge: 3, hot: 1, cold: 1, in: 1, the: 1, pot: 1, nine: 1, days: 1, old: 1 }
 

There’s probably a better way to initialize the tally object than

if (!(item in tally)) {
  tally[item] = 0
}
tally[item] += 1;

(This works more or less like a defaultdict in python.)

But I find the third line in things like this:

function count(arr){
  return arr.reduce(function(m,e){
    tally[item] = (+tally[item] || 0)+1; 
    return tally
  },{});
}

…difficult to read.

Anyway this whole business isn’t that much better than:

var count = function (sequence) {
    var tally = {};
    sequence.forEach(function (item) {
        if (!(item in tally)) {
            tally[item] = 0
        }
        tally[item] += 1;
    })
    return tally;
}

Except, I guess, that a reduce could be used in a function chain.

Brendan pointed out:

console.log(count('watch out for this'.split(' ')))
Object { watch: "function watch() {
    [native code]
}1", out: 1, for: 1, this: 1 }

This is because Object.prototype has a method called .watch() in Firefox. Lame. So you can do this:

var count = function (sequence) {
  return sequence.reduce(function (tally, item) {
    if (!(tally.hasOwnProperty(item))) {
      tally[item] = 0
    }
    tally[item] += 1;
    return tally
  }, {})
}
console.log(count('watch out for this'.split(' ')));

Which works, but then you write over .watch(), which is deprecated anyway. But still. Lame.