Vfrdtyky

Looking at the code for React.Children.map, it looks like the function takes up to 3 args.

The api doc for React.Children.map shows

React.Children.map(children, function[(thisArg)])

If it can take 3 args I expected to see 2 commas in there somewhere. How should I be reading that notation?

Specifically, what does function[(thisArg)] mean?

function[(thisArg)] isn't a standard denotation. As the documentation states, map

Invokes a function on every immediate child contained within children with this set to thisArg.

Children.map signature is similar to JavaScript Array.prototype.map. There are 3 arguments, 3rd argument is optional this context for callback function if needed.

Children.map is documented better in source code:

/**

 * Maps children that are typically specified as `props.children`.

 *

 * See https://reactjs.org/docs/react-api.html#reactchildrenmap

 *

 * The provided mapFunction(child, key, index) will be called for each

 * leaf child.

 *

 * @param {?*} children Children tree container.

 * @param {function(*, int)} func The map function.

 * @param {*} context Context for mapFunction.

 * @return {object} Object containing the ordered map of results.

 */

function mapChildren(children, func, context) {...}