Promise plus number returns promise

Question

I have a simple code

async function foo() {
  const b = bar()
  return 2 + b
}

async function bar() {
  return 2
}

(async() => {
  console.log(typeof foo())
})()

and it logs object. Not NaN. How does it happen that number + object -> object?

From what I remember from + spec if one of the operands is a primitive and second is an object then object should be converted to primitive. In this case with .valueOf() method

Answer 1

It think it's because async functions are not resolved yet in your case, becaue there is no await. So you are getting promise objects instead of your result.

see these cases:

async function foo () {
    const b = await bar() //await result
    return 2 + b
}

async function bar () {
    return 2
}

;(async () => {
    console.log(typeof await foo()) //number
})()

async function foo () {
    const b = bar() //no await
    return 2 + b
}

async function bar () {
    return 2
}

;(async () => {
    console.log(typeof await foo()) //string
})()

Answer 2

The functions foo and bar return a promise, so, you're getting the type of promise (Object)

Probably you want to compare the result, so you need to wait for promise resolves the concatenation:

let result = await foo();
             ^

async function foo() {
  const b = bar() // Here you need to wait as well.
  return 2 + b; // 2 + "[object Promise]"
}

async function bar() {
  return 2
}

(async() => {
  let result = await foo();
  console.log(result);
  console.log(typeof result);
})()

Now, to get the NaN value you need to convert to number:

async function foo() {
  const b = bar();  // Here you need to wait as well.
  return Number(2 + b); // <- Try to conver to number
}

async function bar() {
  return 2
}

(async() => {
  let result = await foo();
  console.log(result);
  console.log(typeof result);
})()