Value of 'arguments' of fat arrow function inside callback giving unexpected values

Question

See the below snippets:

Snippet #1:

let fs = require("fs");
fs.readFile(process.argv[2], "utf8", (error, data) => console.log(arguments));

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Snippet #2:

let fs = require("fs");
fs.readFile(process.argv[2], "utf8", (error, data) => console.log(error, data));

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Expected log: Values of (error, data), for example like:

null 'console.log("HELLO WORLD");\r\n'

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When you try both these snippets, you will find that the Snippet #1 executes and logs some unexpected values for console.log(arguments) but console.log(error, data) logs proper values; values of (error, data).

Why and What was the value that is being logged for Snippet #1?

Answer 1

No binding of arguments

Arrow functions do not bind an arguments object Thus, arguments is simply a reference to the name in the enclosing scope.

From: MDN - Arrow functions

If you wish to use variadic arguments inside an arrow function, use the rest parameters syntax:

fs.readFile(process.argv[2], "utf8", (...args) => console.log(args));

Answer 2

@Tamas is smack on,

But a little tip for the OP, if you ever wonder what new javascript features from ES6 etc are doing. I paste the code into Babeljs.io try it out bit.

eg. Your code ->

https://babeljs.io/repl/#?babili=false&evaluate=true&lineWrap=false&presets=es2015%2Ces2015-loose%2Ces2016%2Ces2017%2Clatest%2Creact%2Cstage-0%2Cstage-1%2Cstage-2%2Cstage-3&code=fs.readFile(process.argv%5B2%5D%2C%20%22utf8%22%2C%20(error%2C%20data)%20%3D%3E%20console.log(arguments))%3B%0D%0A

and then @Tamas code ->https://babeljs.io/repl/#?babili=false&evaluate=true&lineWrap=false&presets=es2015%2Ces2015-loose%2Ces2016%2Ces2017%2Clatest%2Creact%2Cstage-0%2Cstage-1%2Cstage-2%2Cstage-3&code=fs.readFile(process.argv%5B2%5D%2C%20%22utf8%22%2C%20(...args)%20%3D%3E%20console.log(args))%3B