I expected the promise handler to log the promise p1 (not the value "A") since console.log is called with p1 directly. However, it somehow logs "A". How is the promise p1 automatically resolved to "A" without then being called on it ? For example, console.log(p1) does not output "A" directly as is expected. Is something going on behind the scenes ?
var p1 = new Promise(function(resolve, reject) {
resolve("A");
});
var p2 = new Promise(function(resolve, reject) {
resolve(p1);
});
p2.then(function(v) {
console.log(v)
});EDIT: I understand that calling
p1.then((v) => return v))
returns a new promise that is fulfilled with the value v. Unless Im seriously missing something here, the "p1" reference in the second promise constructor should have been passed directly to console.log, resulting in the block
var p2 = new Promise(function(resolve, reject) {
resolve(p1);
});
p2.then(function(v) {
console.log(v)
});
becoming
console.log(p1).
Since console.log is called directly with p1, NOT the result of p1.then(...), p1 should not be resolved into the value "A" in the same way that printing that a another program
var promise = new Promise(function(resolve, reject) {
resolve("B")
})
console.log(promise)
does not result in the string "B".
EDIT2: I had a misconception that the resolve parameter passed to the executor is a wrapper for unfulfilled function, which caused me tons of confusion. Check out Why does the Promise constructor require a function that calls 'resolve' when complete, but 'then' does not - it returns a value instead? for more details.
Resolving a promise to another promise will automatically make it wait for the other promise's result.
This is what makes promises chainable (returning further promises in then() callbacks).