At times, I delve into JavaScript code on MDN and come across some confusing syntax like [, thisArg]... for instance,
arr.map(callback(currentValue[, index[, array]])[, thisArg])
In this scenario, I am aware that a callback function is required. But what exactly do the square brackets signify? Why is there a comma when nothing comes before it?
When you see something in brackets in programming, it typically denotes an optional argument. If you decide to use that optional argument, make sure to separate it with a comma from the previous argument.
For example:
arr.map(callback(currentValue[, index[, array]])[, thisArg])
can be written for clarity as:
arr.map(
callback(currentValue[, index[, array]])
[, thisArg]
)
This notation means that the callback function can take 1, 2, or 3 arguments. The .map method requires the callback function as the first argument and optionally accepts a second argument (the thisArg) as well.
Kaiido points out that in the case of Array.prototype.map, even the currentValue is considered optional. It's unusual but technically possible to use .map without any arguments at all:
const arr = [3, 4];
const newArr = arr.map(() => 999);
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