Can you explain me this code ? specially the condition (count[i]||0)

uniqueCount = ["a","b","c","d","d","e","a","b","c","f","g","h","h","h","e","a"];
var count = {};
uniqueCount.forEach(function(i) { count[i] = (count[i]||0) + 1;});
console.log(count);

Here is the code again, but formatted using prettier.io

var uniqueCount = [
  "a",
  "b",
  "c",
  "d",
  "d",
  "e",
  "a",
  "b",
  "c",
  "f",
  "g",
  "h",
  "h",
  "h",
  "e",
  "a"
];
var count = {};
uniqueCount.forEach(function(i) {
  count[i] = (count[i] || 0) + 1;
});
console.log(count);

The part your specifically asking about is this snippet:

(count[i] || 0) + 1;

Ok so lets break down the 2 parts:

(/*stuff*/) + 1

Just like normal math, stuff in () is executed first, along with being executed left to right. so whatever is in the () will be added with + 1. Nothing fancy here.

Now lets dive into what is in (), and start getting into some “odd” quirks of JS that can be useful, or straight up confusing.

(count[i] || 0)

So the first part is a lookup against the count object with the property i.
So for the first item in the array (‘a’) we essentially are doing this:

count['a'] = (count['a'] || 0 ) + 1;

which assigns count['a'] the value of (count['a'] || 0) + 1, which is just the value of count['a'] or 0. So if the value at count['a'] is “truthy”, then we use it, otherwise we substitute 0.

Now lets jump into the details of how the || logical operator works.
Here’s the definition from mdn:

Logical OR ( || )
example: expr1 || expr2
If expr1 can be converted to true , returns expr1 ; else, returns expr2 .

This means is **if the first value is “truthy” then we return it, otherwise return the 2nd value.
Here is an example:

const expression1 = 'strings are true!';
const expression2 = null;
const expression3 = expression1 || expression2;
console.log(expression3); // strings are true!

const expression4 = expression2 || expression1;
console.log(expression4); // strings are true!

const expression5 = false;
const expression6 = undefined;
const expression7 = expression5 || expression6;
// the '' + will cast the value to a string so we can see it
console.log('' + expression7); //undefined

You might of expected the last example to return false since both undefined and false are falsy values, but that isn’t how the || operator works. The same could be said for how the && operator works, but obviously it handles logic, and thus what is returns differently. (you can read more here)


So essentially || is used to “pick” between the left side or right side for the truthy value, or the last falsy value (as 0 is falsy, but since its last its “used”)

2 Likes

very well explained!!! respect :vulcan_salute: