在类中使用const和arrow函数时出现SyntaxError

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!

Answer 1

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.