I'm learning promises and I'm trying to chain them using the following code. In this, .then()
gets executed first:
function myPromise1()
{
console.log("Promise", 1);
return Promise.all([
new Promise((resolve, reject) => {
setTimeout(() => {
console.log("This should be before 'Yay!' for 1");
resolve(1);
}, 3000);
}),
new Promise((resolve, reject) => {
setTimeout(() => {
console.log("This should be before 'Yay!' for 2");
resolve(2);
}, 3000);
})
]);
}
function myPromise2() {
console.log("Promise", 2);
var arr = [1,2,3,4];
return Promise.all(arr.map(i => {
new Promise((resolve, reject) => {
setTimeout(() => {
console.log("This should be before 'Yay!' for i= " + i);
resolve("Success!" + i);
}, 3000)
})
}));
}
function anotherPromise(val) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(val);
}, 3000);
});
}
myPromise2()
.then((msg) => {
console.log("Yay! " + msg);
return anotherPromise("Awesome!");
})
.then((msg2) => {
console.log("This should be Awesome:", msg2);
})
.catch((err) => {
console.log("Uh no, this is a lie --->", err);
});
console.log("This will print as second line after the method name, even though its the last line.");
This is the output:
Promise 2
This will print as second line after the method name, even though its the last line.
Yay! ,,,
This should be before 'Yay!' for i= 1
This should be before 'Yay!' for i= 2
This should be before 'Yay!' for i= 3
This should be before 'Yay!' for i= 4
This should be Awesome: Awesome!
Process finished with exit code 0
As you can see, .then()
gets called before and hence the values are not returned.
However, if I call myPromise1()
instead, I get the desired output. The values are returned too and .then()
gets called after both the promises.
How can I fix the issue with myPromise2()
?
You are forgetting to return from map
method here:
function myPromise2() {
console.log("Promise", 2);
var arr = [1,2,3,4];
return Promise.all(arr.map(i => {
/* missing return ---> */ new Promise((resolve, reject) => {
setTimeout(() => {
console.log("This should be before 'Yay!' for i= " + i);
resolve("Success!" + i);
}, 3000)
})
}));
}
function myPromise1() {
console.log("Promise", 1);
return Promise.all([
new Promise((resolve, reject) => {
setTimeout(() => {
console.log("This should be before 'Yay!' for 1");
resolve(1);
}, 3000);
}),
new Promise((resolve, reject) => {
setTimeout(() => {
console.log("This should be before 'Yay!' for 2");
resolve(2);
}, 3000);
})
]);
}
function myPromise2() {
console.log("Promise", 2);
var arr = [1, 2, 3, 4];
return Promise.all(arr.map(i => {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log("This should be before 'Yay!' for i= " + i);
resolve("Success!" + i);
}, 3000)
})
}));
}
function anotherPromise(val) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(val);
}, 3000);
});
}
myPromise2()
.then((msg) => {
console.log("Yay! " + msg);
return anotherPromise("Awesome!");
})
.then((msg2) => {
console.log("This should be Awesome:", msg2);
})
.catch((err) => {
console.log("Uh no, this is a lie --->", err);
});
console.log("This will print as second line after the method name, even though its the last line.");