SyntaxError when using const and arrow function inside a class

Question

I'm working through some CodeWars katas and I would like to know: For what reason does this syntax work...

class SmallestIntegerFinder {
  findSmallestInt(args) {
    return Math.min(...args)
  }
}

...but this other does not?

class SmallestIntegerFinder {
  const findSmallestInt = args => Math.min(...args)
}

This is the error trace printed:

const findSmallestInt = args => Math.min(...args)
      ^^^^^^^^^^^^^^^
SyntaxError: Unexpected identifier
    at createScript (vm.js:56:10)
    at Object.runInThisContext (vm.js:97:10)
    at Object.<anonymous> ([eval]-wrapper:6:22)
    at 
    at evalScript (bootstrap_node.js:353:27)
    at run (bootstrap_node.js:122:11)
    at run (bootstrap_node.js:389:7)
    at startup (bootstrap_node.js:121:9)
    at bootstrap_node.js:504:3

I haven't had any problem using the second syntax in other katas, but this is the first time I use it inside a class, so I guess it has something to do with JavaScript classes that I haven't learnt yet or something else that is escaping me at the moment.

Thanks in advance!

Because a class body is not a block that could hold arbitrary statements? — Bergi
– Bergi, Commented Feb 20, 2018 at 1:15
Would you have expected const … = … to work inside an object literal? — Bergi
– Bergi, Commented Feb 20, 2018 at 1:16

djfdev · Accepted Answer · 2018-02-20 01:23:56Z

4

It's because that's not valid syntax. Within a class, you do not use variable bindings like const, let, or var to define members. The syntax is nice and simple.

There is a proposal, however, that will allow class fields, similar to your second example. However, you would still not use const, let, or var to define these either.

Edited: thanks @Bergi for pointing out that class/instance method syntax isn't sugar for anything, that's just how you define methods.

edited Feb 20, 2018 at 1:23

answered Feb 20, 2018 at 1:09

djfdev

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2 Comments

Bergi Over a year ago

Actually, the method definition syntax is not a shorthand for anything, it's the only valid syntax inside a class body. Notice that in contrast to object literals, there are no commata either.

DavSanchez Over a year ago

Thank you all! I guess my error was considering that functions and class methods could use the same syntax

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