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JavaScript Asynchronous Programming with JavaScript Understanding Promises Create a Promise

Jesus Olaiz
Jesus Olaiz
8,259 Points

Im confused on how the callbacks are working here

  const myPromise = new Promise( (resolve, reject) => {
    setTimeout(() => {
    }, 1000);


    .then(a => a+5 )
    .then(b => b*6)
    .then( val => console.log(`You got ${val}`))

I know how to make it work, but there is something about the logic that has been just making me go kind of crazy.

    .then(a => a+5 )

Written out in a pyramid of doom I can still follow the code pretty well and make sense of how the function is getting the result of the next, but here I just cant make sense as to how the engine knows that the result applies to the variable or parameter a. Like I said, I can make a promise work but I would love a bit more in depth explanation of what is happening here so I don't feel like I'm just guessing the whole time and some how getting lucky.

Thanks in advance!

1 Answer

Steven Parker
Steven Parker
216,863 Points

What you're seeing is just the nature of .then. The callback function you pass to it has one argument, the fulfillment value. When the callback is invoked, then passes that value to the function.

You might want to take a look at the MDN page on .then.