Is it possible to invoke another function if the initial callback does not yield a Promise?

Question

Is it possible to invoke another function if the initial callback does not yield a Promise?

Exploring The Power of Promise Chaining and came across this code snippet:

Promise.resolve(123)
    .then((res) => {
        console.log(res); // 123
        return 456;
    })
    .then((res) => {
        console.log(res); // 456
        return Promise.resolve(123); // Pay attention to the fact that we are returning a 
Promise
    })

The first function callback does not return a promise, while the subsequent ones do. Is it possible to call then after the statement

.then((res) => { console.log(res);return 456;})

is executed?

javascript typescript ecmascript-6 es6-promise

Answer 1

Answer №1

According to @jaromanda-x,

The then function in JavaScript returns a Promise, allowing for method chaining.

By passing a lambda to then and returning a promise, an equivalent Promise is passed down the method chain. The following code snippet demonstrates asynchronous behavior with the setTimout function.

Simply returning a value from within a then lambda will result in

Promise.resolve(<value returned by whichever handler was called>)

.

For example:

var p2 = new Promise(function(resolve, reject) {
  resolve(1);
});

p2.then(function(value) {
  console.log(value); // 1
  return value + 1;
}).then(function(value) {
  console.log(value + '- This synchronous usage is virtually pointless'); // 2- This synchronous usage is virtually pointless
});

p2.then(function(value) {
  console.log(value); // 1
});

Explore more about Promises on MDN

Answer 2

According to @jaromanda-x,

The then function in JavaScript returns a Promise, allowing for method chaining.

By passing a lambda to then and returning a promise, an equivalent Promise is passed down the method chain. The following code snippet demonstrates asynchronous behavior with the setTimout function.

Simply returning a value from within a then lambda will result in

Promise.resolve(<value returned by whichever handler was called>)

.

For example:

var p2 = new Promise(function(resolve, reject) {
  resolve(1);
});

p2.then(function(value) {
  console.log(value); // 1
  return value + 1;
}).then(function(value) {
  console.log(value + '- This synchronous usage is virtually pointless'); // 2- This synchronous usage is virtually pointless
});

p2.then(function(value) {
  console.log(value); // 1
});

Explore more about Promises on MDN

Is it possible to invoke another function if the initial callback does not yield a Promise?

Answer №1

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