Get unexpected token, looking for explanation

Question

Might ask this on a different channel. I have this:

const reduceList = (list) => {

  return list.filter(Boolean).reduce((a, b, c) => {
    console.log({this});
  });


};

console.log(reduceList([1, 2, 3]));

I get:

console.log({this});
                 ^

SyntaxError: Unexpected token } at new Script (vm.js:74:7) at createScript (vm.js:246:10) at Object.runInThisContext (vm.js:298:10) at Module._compile (internal/modules/cjs/loader.js:657:28) at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10) at Module.load (internal/modules/cjs/loader.js:599:32) at tryModuleLoad (internal/modules/cjs/loader.js:538:12) at Function.Module._load (internal/modules/cjs/loader.js:530:3) at Function.Module.runMain (internal/modules/cjs/loader.js:742:12) at startup (internal/bootstrap/node.js:266:19)

anybody know why that is? I am on Node.js version 11.

Answer 1

Shorthand object initializer syntax requires an Identifier. While this (and other reserved words) is technically an IdentifierName it cannot be used as an Identifier as explained in the ECMAScript spec.

A reserved word is an IdentifierName that cannot be used as an Identifier.

https://www.ecma-international.org/ecma-262/6.0/#sec-reserved-words

So this isn't limited to the this keyword, a similar syntax error can be produced with other reserved words:

{true} // syntax error
{true: true} // valid object literal

Essentially you need to use a named identifier (a variable) for things to work correctly here. It's possible to use a reserved word as a property's name, but not as an Identifier because they are evaluated differently.