Comma in front of square bracket - MDN docs

user_6134029 0
Viewed 164

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

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

I'm surprised there's a square bracket in front of the comma. It looks like an invalid syntax to me.

I would highly appreciate it if somebody could give more information about this notation.

javascript
3 Answers

The square brackets mean that the enclosed parameter is optional.

Refer MDN syntax section for more info.

user_50555030

That means the callback requires a currentValue as the first argument, and index and array are optional arguments.

user_34809100

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.

user_83761840
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