Javascript: Reduce function with ||

Question

Can someone explain the following code? inputWords is supposed to be an array containing various words and this function is supposed to return an array containing the number of times a word appears in inputWords.

ie. var inputWords = ['Apple', 'Banana', 'Apple', 'Durian', 'Durian', 'Durian']

console.log(countWords(inputWords))
// =>
// {
//   Apple: 2,
//   Banana: 1,
//   Durian: 3
// }

I understand what the Reduce function does, but what is resultObj[word] = ++resultObj[word] || 1; doing?

Thanks so much :)

function countWords(inputWords) {
  return inputWords.reduce(function(resultObj, word) {
    resultObj[word] = ++resultObj[word] || 1;
    return resultObj;
  }, {});
}

module.exports = countWords;

It increments resultObj[word] if it exists, otherwise initializes it to 1. — user703016
– user703016, Commented Jun 2, 2014 at 18:14
|| is the logical or operator, will pick the 1 if the left side is a falsy value — juvian
– juvian, Commented Jun 2, 2014 at 18:15
@vladkras reduce is actually the operation of applying a function to aggregate a collection to a single value, in this case an array with many words to an object counting how many times each word appears in the array. The increment is what actually does the counting. — paolobueno
– paolobueno, Commented Jun 2, 2014 at 18:22
Could as well be written as resultObj[word] = (resultObj[word] || 0) + 1 which gets the intention better across. — Bergi
– Bergi, Commented Jun 2, 2014 at 18:23

Tom Fenech · Accepted Answer · 2014-06-02 19:28:57Z

3

The code attempts to assign to a key (that may not yet exist) an incremented value (of a key that may not yet exist), of if that it that is falsey, it assigns a 1.

This is called short-circuit evaluation. Given a = b || c, if b is truthy, c never gets evaluated so a takes on the value of b. If b is falsey, c is evaluated and assigned to a instead. In your case, when the key doesn't exist, ++resultObj[word] is falsey.

In my humble opinion, I think that it would have been a clearer statement of the author's intention if they had instead done:

if (word in resultObj) {
    ++resultObj[word];
}
else {
    resultObj[word] = 1;
}

or even:

resultObj[word] = word in resultObj ? resultObj[word] + 1 : 1;

either of which would have saved you the bother of asking this question.

edited Jun 2, 2014 at 19:28

answered Jun 2, 2014 at 18:56

Tom Fenech

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Comments

paolobueno · Accepted Answer · 2014-06-02 18:18:39Z

1

It's 'defaulting' a non-existent key in resultObj to 1.

Javascript's || will actually return the first (leftmost) truthy value out of a comparison, so for a nonexistant value added one by the preincrement operator such as ++resultObj['banana'] returns NaN, which is falsey, and the || operator will replace it with 1.

Edit: See http://nfriedly.com/techblog/2009/07/advanced-javascript-operators-and-truthy-falsy/

answered Jun 2, 2014 at 18:18

paolobueno

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Comments

Christian Landgren · Accepted Answer · 2015-07-18 13:07:53Z

1

I would write:

resultObj[word] = (resultObj[word] || 0) + 1;

answered Jul 18, 2015 at 13:07

Christian Landgren

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