sneaky-arrow-functions.md

Micro Post: Sneaky Arrow Functions

I was just hacking away on a side project when I wrote this code:

const screencastMock = {
  findOne: () => { }
};

My intention was to make findOne return an empty object literal ({}) but it actually returns undefined.

Can you spot my mistake?

---

Answer

When JavaScript sees () => { }, it interperts the { } part as a block statement instead of an empty object literal:

const screencastMock = {
  findOne: () => { var insideABlockStatement = true; var owned = true; } // valid JavaScript!
};

The block statement doesn't contain a return statement so this is implicitly returned. In this context, this was undefined, hence why findOne returns undefined.

To successfully return an empty object literal, I should have surrounded the empty object literal in parentheses, like this:

const screencastMock = {
  findOne: () => ({})
}

I found this error amusing because I'm well aware of this caveat with arrow functions and yet it still tripped me up 😆! If I wasn't aware of this caveat, who knows how long I could have been stuck here...

I wounder if there's an ESLint plugin available to help avoid this mistake...?

I will answer my own question:
I guess it's because arrow functions expect either an expression or a block.
A fat arrow only knows about block, it doesn't know about object literal. So if you use { } it always treats as a block.
http://exploringjs.com/es6/ch_arrow-functions.html#_arrow-function-syntax

The specs call it "ConciseBody", it's either an expression or function body:
http://www.ecma-international.org/ecma-262/6.0/#sec-arrow-function-definitions

ArrowFunction[In, Yield] :
ArrowParameters[?Yield] [no LineTerminator here] => ConciseBody[?In]

ConciseBody[In] :
[lookahead ≠ { ] AssignmentExpression[?In]
{ FunctionBody }

One thing I don't understand in the spec is what AssignmentExpression is.
Why is it called assignment expression? Since we can do this:
const f = x => x;
in this case the body of the arrow function is just x, it's not an assignment expression.