Comma in front of square bracket - MDN docs

Question

MDN is my primary Javascript resource. I often see a notation like ( currentValue[, index[, array]]) as in:

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Answer 1

It is an invalid JS syntax, and it would throw an error.

However, it is used to denote optional arguments as a pseudo-code.

The parts in the square brackets are considered optional (and in the real code the brackets should be omitted), so, you might call this function like:

let new_array = arr.map(function callback(currentValue) {
    // return element for new_array
})
//or
let new_array = arr.map(function callback(currentValue) {
    // return element for new_array
}, thisArg)
//or
let new_array = arr.map(function callback(currentValue, index) {
    // return element for new_array
})
//or
let new_array = arr.map(function callback(currentValue, index) {
    // return element for new_array
}, thisArg)
//or
let new_array = arr.map(function callback(currentValue, index, array) {
    // return element for new_array
})
//or
let new_array = arr.map(function callback(currentValue, index, array) {
    // return element for new_array
}, thisArg)

and each of them is valid.