javascript - Value of 'arguments' of fat arrow function inside callback giving unexpected values - Stack Overflo

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See the below snippets:

Snippet #1:

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

See the below snippets:

Snippet #1:

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

---

Snippet #2:

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

---

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

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

---

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?

• javascript
• node.js
• ecmascript-6

Share Improve this question Follow asked Nov 5, 2016 at 8:07 Temp O'raryTemp O'rary 5,8381515 gold badges5353 silver badges114114 bronze badges 2

• Should those snippets be runnable? Coz, require will not work – Rajesh Commented Nov 5, 2016 at 8:09
• See also Arrow function vs function declaration / expressions: Are they equivalent / exchangeable?. – Felix Kling Commented Nov 5, 2016 at 14:59

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2 Answers 2

Sorted by: Reset to default 10

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));

@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

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