The JavaScript Interview Handbook: 100+ Interview Questions/

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Solution Review: Promises

Solution Review: Promises

In this lesson, we will discuss the solutions to the questions in the previous lesson.

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() {
...