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javascriptarraysreduce
Jo Colina
Jo Colina

Reputation: 1914

JavaScript reduce returns object on Array of objects

I have an array of objects, let's say [{x:2, y:3}, {x:5, y:4}] and i call reduce((c, n) => c.y + n.y); on it. It obviouslly returns 7.

However, if the array contains a single object, let's say [{x:2, y:4}] the same reduce call will return the object itself {x:2, y:4}.

Is this normal behaviour? Am I obliged to check if the result is an object and not an number afterwards?

Upvotes: 9

Views: 31316

Answers (5)

Maciej Kozieja
Maciej Kozieja

Reputation: 1865

That's because reduce function can take 1 more argument - default value and if its not specified it takes firs value of array. That's why it works for more than one.

However if you do it like this

let a = [{x:5, y:4}].reduce((c, n) => c + n.y, 0);

console.log(a)

it sums propertly.

Upvotes: 4

DarkArtistry
DarkArtistry

Reputation: 434

Note: If initialValue isn't provided, reduce will execute the callback function starting at index 1, skipping the first index. If initialValue is provided, it will start at index 0.

If the array is empty and no initialValue is provided, TypeError will be thrown. If the array has only one element (regardless of position) and no initialValue is provided, or if initialValue is provided but the array is empty, the solo value will be returned without calling callback.

It is usually safer to provide an initial value because there are three possible outputs without initialValue, as shown in the following example.

Upvotes: 2

Sushanth --
Sushanth --

Reputation: 55740

Did you initialize the accumulator to { y : 0}

If you did not then it will return the original object.

let sum = data.reduce((c, n) => {
  return { "y" : c.y + n.y };
}, { y : 0 });

Upvotes: 3

Bergi
Bergi

Reputation: 664385

Yes, this is the normal behaviour of reduce when you don't pass an initial value for the accumulator (which you always should). Your code doesn't work as expected on any arrays other than those with two objects.

Go for

arr.reduce((acc, el) => acc + el.y, 0)

Upvotes: 20

trincot
trincot

Reputation: 350147

It is intended behaviour, when you don't provide an initial value (second argument to reduce). From MDN:

If the array is empty and no initialValue was provided, TypeError would be thrown. If the array has only one element (regardless of position) and no initialValue was provided, or if initialValue is provided but the array is empty, the solo value would be returned without calling callback.

It comes with the following advise:

It is usually safer to provide an initial value because there are three possible outputs without initialValue.

So write:

reduce((c, n) => c.y + n.y, { y: 0 });

Upvotes: 5

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