Transforming Values

Calling return within then()

Promise results can be transformed by calling the return statement within the then() callback. This will cause the then() method to return a new Promise with the transformed result.

Notice how a new Promise is created with a transformed result using the return statement within the then() callback:

let promise = Promise.resolve("hello");

let promise2 = promise.then(result => { 
    console.log(result) //logs "hello"
    return result + " world" //adds " world" to the result and sets this as the new fulfillment value of promise2
});

promise2.then(result => {
    console.log(result); //logs "hello world"
});

Chaining Transforms

Several transforms can be chained together using multiple then() method calls.

Notice how promise results are transformed using multiple then() methods calls:

let promise = Promise.resolve([1,2,3,4]);

promise.then(result => { 
    console.log(result) //logs [1,2,3,4] 
    return result.map(x => x * x); //squares each value in the array

}).then(result2 => {
    console.log(result2) //logs [1,4,9,16]
    return result2.filter( x => x > 10); //filters out elements that are not larger than 10

}).then(result3 => {
    console.log(result3) //logs [16]
    return result3.toString() + "!!"; //converts result3 to a string and adds "!!"

}).then(result4 => {
    console.log(result4) //logs "16!!"
    return result4;  //returns a promise with "16!!" as the fulfillment value

}).catch(error => {
    console.log(error)
});