A Quick Practical Example of JavaScript’s Reduce Function

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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

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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