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The JavaScript Interview Handbook: 100+ Interview Questions

Solution Review: Promises

Explore how to solve asynchronous programming challenges using JavaScript promises. Understand promise creation, chaining with .then, error handling with .catch, and how to interpret promise outputs. This lesson helps you confidently manage promise flows in coding interviews.

We'll cover the following...

Question 1: Solution review #

In the previous lesson, you were given the following code:

Javascript (babel-node)
var promise = func1();
promise
.then(function(result1) {
    console.log(result1);
    return func2();
})
.then(function(result2) {
    console.log(result2);
    return result2%10;
})
.then(function(result3) {
    console.log(result3);
});
function func1() {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            resolve("Hello");
        }, 1000);
    });
}
function func2() {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            resolve(50);
        }, 1000);
    });
}

For the code above, you had to answer the following question:

Explanation #

Run the code below to see the solution.

Javascript (babel-node)
var promise = func1();
promise
.then(function(result1) {
    console.log(result1);
    return func2();
})
.then(function(result2) {
    console.log(result2);
    return result2%10;
})
.then(function(result3) {
    console.log(result3);
});
function func1() {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            resolve("Hello");
        }, 1000);
    });
}
function func2() {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            resolve(50);
        }, 1000);
    });
}

As you can see, the correct option is A. Let’s discuss the code to understand the answer.

On line 1, there is a variable promise that stores the promise returned from the function func1.

var promise = func1();

Once, this promise is received, there is a series of .then statements following it.

On line 5, the first .then statement is called. It has a callback as its first parameter for when the promise is resolved. Inside this callback, there is a console.log statement outputting the result received from the first promise, that is, the result from func1.

.then(function(result1) {
    console.log(result1);
    //..
})

If you follow the code, you’ll find the definition of func1 on line 19. As you can see, it returns a promise with the result Hello after a delay of 1000 milliseconds.

function func1() {
...