Why do I need to write "function(value) {return my_function(value);}" as a callback in node.js?

user_224254 0
Viewed 2941

totally new to JS so please forgive if this is mind-bogglingly obvious.

Suppose I want to filter a list of strings with a function f that maps string -> bool. This works:

filteredList = list.filter(function(x) { return f(x); })

This fails:

filteredList = list.filter(f)

Why???

Code example:

 ~/projects/node (master)$ node
> var items = ["node.js", "file.txt"]
undefined
> var regex = new RegExp('\\.js$')
undefined
> items.filter(regex.test)
TypeError: Method RegExp.prototype.test called on incompatible receiver undefined
    at test (native)
    at Array.filter (native)
    at repl:1:8
    at REPLServer.self.eval (repl.js:110:21)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)
    at ReadStream.onkeypress (readline.js:99:10)
> items.filter(function(value) { return regex.test(value); } )
[ 'node.js' ]
>
node.js callback javascript
3 Answers

As others have said, this is undefined when the function reference is called. I'd like to offer a non-reflexive, slightly verbose, alternative:

items.filter(RegExp.prototype.test.bind(/regex/g));
user_41356700
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