Does .resolve() either call a function, resolve to value or accept a value, or is the promise doing these things?

Question

Resolve has a lot of varied uses.. 
Some places (https://teamtreehouse.com/library/asynchronous-programming-with-javascript/create-a-promise) it's not passed anything at all but still 'returns' a promise object. I.e: resolve()
Some places (https://teamtreehouse.com/library/asynchronous-programming-with-javascript/promises-review) it's supplied a variable as an argument. I.e. resolve(value)
Other places (https://teamtreehouse.com/library/asynchronous-programming-with-javascript/using-fetch#transcript) it "returns a response object" or " returns a promise that resolves to a response object" (not sure why the terminology is different or if they mean different things). I.e. resolve(data)
Here's a RESOLVE(value) Example:

const value = 5;

This promise passes value along with chained .then() methods.
js
const mathPromise = new Promise( (resolve, reject) => {
  setTimeout( () => {
    // resolve promise if 'value' is a number; otherwise, reject it
    if (typeof value === 'number') { //❓ When is this passed into the promise for evaluation?
      resolve(value);
    } else {
      reject('You must specify a number as the value.')
    }
  }, 1000);
});

Aside: When the promise below is called, how is the initial value for the value variable required in addFive passed into the promise in first place?
js
function addFive(n) {
  return n + 5;
}
function double(n) {
  return n * 2;
}
function finalValue(nextValue) {
  console.log(`The final value is ${nextValue}`);
}
mathPromise
  .then(addFive)
  .then(double)
  .then(finalValue)
  .catch( err => console.log(err) )

Here's a RESOLVE(callback) Example:
js
new Promise( resolve => { 
    const xhr = new XMLHttpRequest();
    xhr.open('GET', url);
    xhr.onload(() => resolve(JSON.parse(xhr.responseText)));
    xhr.send();
  });

Here's another unique Example where the callback is not in resolve, but in the .then() method!!
js
fetch(astrosURL)
    .then( response => response.json())

Steven Parker · Accepted Answer

Be careful not to confuse the resolve parameter of the function passed when creating a Promise with the .resolve() static method which creates a new promise in an already-resolved state (or resolves and existing promise).  For more information on the first, see the MDN page for Promise (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) and for the second, see the MDN page for Promise.resolve() (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve). 
In the sample code, the value passed to addFive comes from the fulfillment of mathPromise, which directly accesses the global variable value when it calls resolve(value) in its code.  And the answer to "When is this passed into the promise for evaluation?" is when the setTimeout completes, 1 second after mathPromise is created.
And in the final example, there's no access to the creation of the promise returned by fetch, so implementing the callback using .then() is the only option.
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Samuel Kleos

Samuel Kleos

Does .resolve() either call a function, resolve to value or accept a value, or is the promise doing these things?

1 Answer

Steven Parker

Steven Parker

Samuel Kleos

Samuel Kleos

Steven Parker

Steven Parker