Creating a promise chain in a for loop
See Resolve promises one after another (i.e. in sequence)?.
Using i of a
:
function createChain() {
let a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0];
let chain = Promise.resolve();
for (let i of a) {
chain = chain.then(()=>getProm(i))
.then(Wait)
}
return chain;
}
Using a.forEach()
function createChain() {
let a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0];
let chain = Promise.resolve();
a.forEach(i =>
chain = chain.then(()=>getProm(i))
.then(Wait)
);
return chain;
}
Using a.reduce()
function createChain() {
let a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0];
return a.reduce((chain, i) =>
chain.then(()=>getProm(i))
.then(Wait),
Promise.resolve()
);
}
You have to assign the return value of .then
back to chain
:
chain = chain.then(()=>getProm(i))
.then(Wait)
Now you will basically be doing
chain
.then(()=>getProm(1))
.then(Wait)
.then(()=>getProm(2))
.then(Wait)
.then(()=>getProm(3))
.then(Wait)
// ...
instead of
chain
.then(()=>getProm(1))
.then(Wait)
chain
.then(()=>getProm(2))
.then(Wait)
chain
.then(()=>getProm(3))
.then(Wait)
// ...
You can see that the first one is actually a chain, while the second one is parallel.