Elliot Elliot - 4 months ago 20
Javascript Question

JS ES6 Promise Chaining

I'm trying to learn how to use promises, but am having trouble comprehending the chaining. I assume that with this code, both promises will run. Then when I call test.then() it should know that test has resolved and pass the resolve data to then().

Once that function finishes, it goes onto the next then(), repeating the same process with the test2 promise.

However, I can only get it to print out the first promise results, not the second. Any ideas what is missing here?

var test = new Promise(function(resolve, reject){
resolve('done1');
});

var test2 = new Promise(function(resolve, reject){
resolve('done2');
});

test
.then(function(data) {
console.log(data);
})
.then(test2)
.then(function(data) {
console.log(data);
});

Answer

Your first .then call is returning undefined, whereas any subsequent .then is expecting a returned promise. So you'd need to change your code to:

var test = new Promise(function(resolve, reject){
    resolve('done1');
});

var test2 = new Promise(function(resolve, reject){
    resolve('done2');
});

test
.then(function(data) {
    console.log(data);
    return test2;
})

.then(resultOfTest2 => doSomething)
.then(function(data) {
console.log(data);
});