How to fix this function that counts items based on their occurrence?

Question

I have the following array:

const sales = [
  { name: 'a', count: 0 },
  { name: 'a', count: 0 },
  { name: 'b', count: 0 }
]

I want to count their occurrence:

const result = []
sales.forEach(sale => {
  if (result.includes(sale)) { // check if the item is in the new array
    sale.count += 1 // if it is, add one to count
  } else {
    result.push(sale) // if not, put the item in the array 
  }
})

But the if statement never returns true. Why is this and how to fix it?

"[How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/)". — Uwe Keim, Jan 03 '20 at 06:17
You never can have two same objects in that array of object literals, though they might have the same content. — Teemu, Jan 03 '20 at 06:20
`{ name: 'a', count: 0 } === {name: 'a', count: 0 }` will return `false`, because although those two objects contain the same properties and values they do not represent the same object in memory, which is what you are testing for (and how I imagine `.includes` works). You probably want to compare the `name` property instead. — Alexander Nied, Jan 03 '20 at 06:22
Does this answer your question? [How to determine equality for two JavaScript objects?](https://stackoverflow.com/questions/201183/how-to-determine-equality-for-two-javascript-objects) — starball, Nov 11 '22 at 05:04

uminder · Accepted Answer · 2020-01-03T06:53:34.630

1

There exists a good answer already at https://stackoverflow.com/a/201471/2358409 that explains why your solution doesn't work.

To make it work, you could transform individual objects in the array to string, and use a Set to count distinct elements.

const sales = [
  { name: 'a', count: 0 },
  { name: 'a', count: 0 },
  { name: 'b', count: 0 }
]

const count = new Set(sales.map(s => JSON.stringify(s))).size;
console.log(count)

edited Jan 03 '20 at 06:53

answered Jan 03 '20 at 06:40

uminder

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It looks like OP wants to store the count of the similar objects in a property of the objects themselves, not just count the occurrences. – Teemu Jan 03 '20 at 06:46
In his question he writes "I want to count their occurrence". – uminder Jan 03 '20 at 06:48
Yes, but in the code, there's `sale.count += 1`, which indicates where the output should actually occur. – Teemu Jan 03 '20 at 06:49

How to fix this function that counts items based on their occurrence?

1 Answers1