CODEWITHSUNDEEP
javascriptreduce
Larynx
Larynx

Reputation: 408

Counting common elements of two Arrays in JavaScript (reduce function)

I met an unexpected result when I count the number of common elements in two arrays. When I use reduce function, it doesn't work. But filter version returns correct result. I want to know what's wrong with my reduce version.

var ls = [1, 2, 3, 4, 5];
var ms = [4, 5, 6, 1, 2];

console.log(ls.reduce((acc, e) => (ms.includes(e) ? 1 + acc : 0), 0));
// 2, incorrect

console.log(ls.filter(e => ms.includes(e)).length);
// 4, correct

Upvotes: 1

Views: 704

Answers (2)

Jamiec
Jamiec

Reputation: 136114

Because in your reduce version, when the element is not found, you reset the acc back to zero instead of returning it as is

var ls = [1, 2, 3, 4, 5];
var ms = [4, 5, 6, 1, 2];

console.log(ls.reduce((acc, e) => (ms.includes(e) ? 1 + acc : acc), 0));
// -----------------------------------------------------------^ here
// 4, now corrected

Upvotes: 2

apulcino
apulcino

Reputation: 11

When your callback function that's the first parameter of the reduce function hits the third item in the ls array, "3", it finds that it's not a member of the ms array. This causes the ternary operator to return the right hand expression 0, which resets your accumulator variable, acc. This will restart the count at 0.

Instead, you should return the current value of the accumulator variable instead of 0 like this:

console.log(ls.reduce(acc,e) => (ms.includes(e) ? 1 + acc: acc), 0));

That will give you the correct count of 4 matching elements.

Upvotes: 0

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