Parsing the javascript reduce documentation

Question

In Mozilla's Reduce docs page I found the syntax below:

arr.reduce(callback[, initialValue])

The square brackets throw me off.

I have code like this that works:

const selectedItems = selectedItemsList.reduce((itemObject, item) => {
  return {
    ...itemObject,
    [item]: this.props.areasList.find(area => area.id === item)
  };
}, {});

I do not use square brackets. I provide a callback and an initial value, separated by a comma. Can anyone elaborate on why the doc shows the syntax that it does for reduce? I only figured out the code in this post that I have by looking at other examples.

it denotes optional parameters – floor Feb 13 '18 at 16:33 — floor, Feb 13 '18 at 16:33
Spec page? It's a documentation page. – Bergi Feb 13 '18 at 16:36 — Bergi, Feb 13 '18 at 16:36

score 1 · Accepted Answer · answered Feb 13 '18 at 16:32

1

The square brackets mean that the initialValue parameter is optional, i.e. you don't have to provide it. You shouldn't actually place these brackets in your code.

answered Feb 13 '18 at 16:32

Michał Perłakowski

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Parsing the javascript reduce documentation

1 Answers1