Javascript pattern help needed

This is actually from a typescript file, but can someone explain this pattern to me:
public filter(filter?: (index: number) => boolean): void {
	this._filter = filter;
	if (this._data) {

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So the name of function becomes a function param ('filter?'), and also is a class within the function ('=filter;')?
I'm trying to get my head round that.
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Steve BinkCommented:
It means that the first parameter is optional (?), and should be a function which itself takes a single numeric parameter and returns a boolean.

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