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javascriptpromisesettimeout

What value is passed by Promises to then when setTimeout is used inside then and why it increment?

When you run this code

Promise.resolve(console.log('resolved'))
  .then(pass => setTimeout((pass) => console.log(`1 then pass=${pass}`), 1000, pass))
  .then(pass => setTimeout((pass) => console.log(`2 then pass=${pass}`), 1000, pass))
  .then(pass => setTimeout((pass) => console.log(`3 then pass=${pass}`), 1000, pass))
  .then(pass => setTimeout((pass) => console.log(`4 then pass=${pass}`), 1000, pass))
  .then(pass => console.log(`5 then pass=${pass}`))
  .then(pass => console.log(`6 then pass=${pass}`))

you get this in console

resolved
5 then pass=4
6 then pass=undefined
1 then pass=undefined
2 then pass=1
3 then pass=2
4 then pass=3

So the question is where this pass variable gets it's value? Where it is stored as window.pass == undefined? Why it increment? Why it finaly gets undefined as we stop using setTimeout?

Solution

  • setTimeout returns a TimerID, so that's the value being passed around.

    1 and 6 are the results of console.log calls which return undefined.