How to Return an Array from Async Function in Node.js ?
Last Updated : 21 Jun, 2024
Asynchronous programming in Node.js is fundamental for handling operations like network requests, file I/O, and database queries without blocking the main thread. One common requirement is to perform multiple asynchronous operations and return their results as an array. This article explores how to achieve this using async functions in Node.js.
Understanding Async Functions
Async functions in Node.js allow you to write asynchronous code that looks and behaves like synchronous code. An async function always returns a promise, and the value returned by the function is wrapped in a resolved promise.
async function example() {
return 'Hello, World!';
}
example().then(console.log);
// Outputs: Hello, World!Returning an Array from an Async Function
To return an array from an async function, you typically want to collect results from multiple asynchronous operations and return them as an array. Let’s walk through several approaches to achieve this.
Example 1: Below is the code in which we call the print function. We define the array in this function(in this case asynchronous), pass it to another async function sort. The sort function then sorts the array and returns the array, and then we display the array from the print function.
Node // index.js const sort = async (arr) => { try { let i, j, temp; // Using bubble sort to sort the array for (i = 0; i < arr.length; i++) { for (j = i + 1; j < arr.length; j++) { if (arr[i] > arr[j]) { // The await prevents the // execution of next line // until this line is executed temp = await arr[i]; arr[i] = await arr[j]; arr[j] = await temp; } } } // Returns the sorted array return arr; } catch (e) { // Display the error in case // anything wrong happened console.log(e) } } const print = async () => { // Use try-catch block to handle error try { // Define and initialize an array let arr = [17, 90, 13, 10, 76] // Call the "sort" function passing // the array and store the result in // "sortedArray", await will prevent // the execution of next line until // this line is executed. const sortedArray = await sort(arr) // Display sortedArray to check if // everything works fine console.log(sortedArray) } catch (e) { // Display the error in case // anything wrong happened console.log(e) } } // Calling print() is called print(); Run the index.js file using the following command:
node index.js
Output:
[ 10, 13, 17, 76, 90 ]
Example 2: Now let's see the example of returning an array from an async function which has been declared in that async function. The function will return an array of all the prime numbers that are less than a given number N.
Node // index.js const prime = async (N) => { try { const arr = [] // Iterate from 2 to N-1 to check // which numbers are prime for (var i = 2; i < N; i++) { let j = 2 while (j < i) { // Check if the number is // divisible by any number // except 1 and itself if (i % j == 0) { break } j++; } if (j == i) { // If this condition holds // true then it means that // the number is not // divisible by any number // except 1 and itself. // Therefore, it is a prime // number and add this number // to the array. arr.push(j) } } // Return the array return arr } catch (e) { // Display the error in case // anything wrong happened console.log(e) } } const findPrime = async () => { try { let N = 20 // Call prime() passing argument // N which is equal to 20 and // store the result in an array const primeAry = await prime(N) // Print the array console.log(primeAry) } catch (e) { // Display the error in case // anything wrong happened console.log(e) } } // Calling findPrime() method findPrime() Output[ 2, 3, 5, 7, 11, 13, 17, 19 ]
Example 3: When dealing with multiple asynchronous operations, you can use Promise.all to execute them in parallel and collect their results into an array.
Node // index.js async function fetchData() { const promise1 = new Promise(resolve => setTimeout(() => resolve('data1'), 1000)); const promise2 = new Promise(resolve => setTimeout(() => resolve('data2'), 500)); const promise3 = new Promise(resolve => setTimeout(() => resolve('data3'), 2000)); const results = await Promise.all([promise1, promise2, promise3]); return results; } fetchData().then(console.log); Outputs:
[ 'data1', 'data2', 'data3' ]
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