JavaScript arrow functions with and without const

Question

test = (x,y) => {}
vs
const test = (x,y) => {}

In SoloLearn js course, arrow functions are always used with const but they also work without it.

So what is the difference?

Answer 1

Aaron Eberhardt
Great explanation but I'd like to correct something: when not using a keyword var WILL NOT be inserted by the interpreter.
Actually the interpreter makes a global variable as a property of the global object (window in our case)
window.test = (x,y) => {}
https://code.sololearn.com/WIZ01zj1eZsM/?ref=app

Prof. Dr. Zoltán Vass
For that reason NEVER define a variable without keyword (var let const)

Answer 2

Prof. Dr. Zoltán Vass sry, I didn't read the question correctly.
Not using any keyword is basically syntactically wrong, at least in strict mode. Most JS engines will automatically convert
test = () => {}
to
var test = () => {}
More correct would be the good old function declaration:
function test() {}
However with const and let functions are scoped otherwise they are global or semi scoped when using var. Sadly JS is a bit messed up due to some bad decisions and maintaining comparability so it's a bit hard to summarise all differences.
But to summarise all points use
function name() {...}
for global functions that should be accessible everywhere,
const name = () => {...}
for functions that operate in a certain scope or that are only needed in a certain scope and use
let name = () => {...}
for functions you want to reassign later.

Answer 3

Aaron Eberhardt Very clear and detailed explanation, I appreciate it very much!

I tested the code in a mobile console (I have only a smartphone by me now) and it accepted the declaration without var or const - perhaps this caused the confusion.

Answer 4

Prof. Dr. Zoltán Vass yes that's right. Not using a keyword will usually work on most browser. As I said a 'var' will be inserted by the interpreter. But that's just tolerated in order to maintain comparability with old web sites. If you use strict mode by writing
'use strict';
at the top of you document the interpreter won't accept that. The strict mode enforces the syntax a lot more and also runs faster because the code can be better optimised.

Answer 5

Aaron Eberhardt Thanks Aaron! It also works without var (without anything before the function name). Could you also explain the difference between them?

Answer 6

As was said, if do not write then the interpreter write the „var“ for you.