What would a do statement return?

[Explosion-Scratch]

In the README there is an example as such:

let x = do {
  if (foo()) { f() }
  else if (bar()) { g() }
  else { h() }
};

What would the value of x be?

Would it act like a function with return (and if so why have this)? What would the real usage of this?

[nicolo-ribaudo]

That code would be equivalent to this:

let x = (() => {
  if (foo()) { return f() }
  else if (bar()) { return g() }
  else { return h() }
})();

The difference bwtween do expressions and iife (other than the implicit "return") is that you can use control flow statements that affect the outer scope. For example:

function getName(str) {
  const person = do {
    let data = JSON.parse(str);
    if (!data.hasName) return null;
    data.person;
  };
  return person.name;
}

is equivalent to

function getName(str) {
  let didReturn;
  let returnValue;
  const person = (() => {
    let data = JSON.parse(str);
    if (!data.hasName) {
      didReturn = true;
      returnValue = null;
      return;
    }
    return data.person;
  })();
  if (didReturn) return returnValue;
  return person.name;
}

[Explosion-Scratch]

The difference between do expressions and iife (other than the implicit "return") is that you can use control flow statements that affect the outer scope. For example:

function getName(str) {
 const person = do {
   let data = JSON.parse(str);
   if (!data.hasName) return null;
   data.person;
 };
 return person.name;
}

So in this function if !data.hasName null would be returned to from the genName() function, right? Also variables in a do statement act much like a with statment in that the variables and such defined there are scoped to the parent scope, right? So this would work:

function aFunction(){
do {
let scoped = "yay";
}
return scoped
}

but this would not:

function aFunction(){
(() => {
let scoped = "yay";
})()
return scoped;//Error, scoped is not defined
}

[nicolo-ribaudo]

do expressions act exactly as normal block statements.

So in this function if !data.hasName null would be returned to from the genName() function, right?

Yes, exactly.

So this would work:

function aFunction(){
  do {
    let scoped = "yay";
  }
  return scoped
}

No, similarly to how this doesn't work:

function aFunction(){
  {
    let scoped = "yay";
  }
  return scoped
}

However, this would work:

function aFunction(){
  (do {
    var scoped = "yay";
  });
  return scoped
}

exactly as this currently works in JS:

function aFunction(){
  {
    var scoped = "yay";
  }
  return scoped
}

[Explosion-Scratch]

Okay! Thanks!