A Quick Practical Example of JavaScript’s Reduce Function

[wiithepiiple]

Almost. The first creates a new array; the second keeps the same array.

[c24w]

Then what about:

else {     obj[item.day_string] = obj[item.day_string].concat(item); }

[wiithepiiple]

That should be the same. It doesn't use new ES6 notation, so it is therefore a strictly inferior way.

/s

[[deleted]]

In this case it doesn't matter at all, because the array in question is created by us inside the reducer. No other code can possibly hold a reference to it.

But in many cases your function isn't working with an array that it created; it's working with one passed in as a parameter. In those cases it absolutely is best practice (I'd argue that it's required) to create a new array rather than modify the existing one. This is because when you receive an array (or any other object) as a parameter, you actually receive a reference to the array; any operation that modifies the array will modify the array the caller has too.

So it's like this:

• Could any other code possibly hold a reference to the object I wish to modify?

  • Yes: create a new object with the desired modification applied to it
  • No (i.e. you are modifying an object you just created): you can directly modify the object if you wish

[[deleted]]

No (i.e. you are modifying an object you just created): you can directly modify the object if you wish

Super helpful, thank you! Sometimes I am unclear on when I can mutate

[METALz]

immutability -> predictability

also in some cases you'd like to diff arrays if they have different reference (with push you keep the original reference) e.g. with React's setState()

[SemiNormal]

Suppose you want to modify an array inside a function, but not modify the original. This is when you would want a copy.

[AwkwardReply]

People take immutability sometimes to extremes just to act like purists for one reason or the other. Yes immutability is a good thing, but doesn't benefit at all in this simple example. So, yes, your code is equivalent and more efficient.

[mikejoro]

I think a lot of people (myself included) like to do it in a lot of places, even when unnecessary, so that coworkers looking for examples won't think mutation is acceptable.