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Master synchronous and asynchronous programming in JavaScript with this comprehensive guide covering callbacks, promises, async/await, and best practices.
Published on: October 5, 2025
Understanding synchronous and asynchronous in JavaScript is essential for writing efficient code. Synchronous operations execute tasks one after another, blocking the program until each completes, whereas asynchronous operations run tasks independently, enabling non-blocking execution. Mastering these concepts helps manage timing, improve performance, and handle tasks like API calls effectively.
What Is Synchronous and Asynchronous in JavaScript?
Synchronous JavaScript runs tasks one after another, waiting for each to finish. Asynchronous JavaScript lets tasks run independently, handling results later without blocking the main thread.
What Are the Features of Synchronous in JavaScript?
Synchronous JavaScript executes tasks sequentially, blocking further execution until each completes, ensuring predictable, easy-to-debug linear flow.
console.log("Start");
let sum = 2 + 3;
console.log("Sum:", sum);
console.log("End");
What Are the Features of Asynchronous in JavaScript?
Asynchronous JavaScript runs tasks independently in the background, enabling non-blocking execution, improved responsiveness, and efficient concurrent operations.
console.log("Start");
setTimeout(() => {
console.log("Asynchronous task completed");
}, 2000);
console.log("End");
What Are the Core Difference Between Synchronous & Asynchronous?
Synchronous blocks execution sequentially, while asynchronous runs tasks independently, improving responsiveness, performance, and handling complex operations efficiently.
Can Asynchronous JavaScript Improve Server-Side Performance in Node.js?
Yes, asynchronous JavaScript is crucial for high-performance server-side applications. In Node.js, non-blocking I/O allows the server to handle multiple client requests concurrently without waiting for each task to finish.
This results in better scalability and reduced response times. Developers can use async/await, promises, or event-driven APIs to optimize server throughput. It’s particularly effective for database queries, file operations, or external API calls.
In Synchronous JavaScript, code executes sequentially, running one task at a time. Each operation must complete before the next begins, ensuring predictable flow and easier debugging.
Synchronous execution can block the main thread during long-running tasks, potentially causing UI freezes or delays in applications that require responsiveness.
You read about what synchronous JavaScript is under the last header, and I explained what its execution feels like in-depth with a physical analogy.
Now, let’s look at a simple example to see it in action:
Code Example:
console.log("Step 1"); // executes first
console.log("Step 2"); // executes after Step 1
console.log("Step 3"); // executes after Step 2
In synchronous execution, each task runs one after another. The main thread completes the current function before moving to the next, blocking further execution until the current task finishes, even if it involves waiting for an external resource.
When learning about synchronous and asynchronous in JavaScript, it’s important to see how blocking operations work. In Node.js, synchronous methods stop further execution until the task completes, making them easier to understand but less efficient for I/O-heavy tasks.
Code Implementation:
let fs = require("fs")
console.log("Starting read operation synchronously")
let data = fs.readFileSync("./somefile.txt", { encoding: "utf8" })
console.log("File Content:", data)
console.log("Finished read operation synchronously")
Code Walkthrough:
When learning about synchronous and asynchronous in JavaScript, it’s important to understand that not all callbacks are asynchronous. Synchronous callbacks execute immediately and block the function until they finish, helping illustrate how execution flows in a single-threaded environment.
Code Implementation:
function solve(a, b, callback) {
let result = a + b
callback(result) //block other operations in function till callback finishes executing
console.log("Solve operation finished, solution obtained")
}
function callback(result) {
let finalResult = 2 * result
console.log(finalResult)
}
solve(2, 3, callback)")
Code Walkthrough:
Note: Run JavaScript automation testing at scale across 3000+ browsers and OS combinations. Try LambdaTest Now!
In asynchronous JavaScript, code is scheduled to run later, often after the synchronous parts of your script have executed and the main thread is free.
Tasks don’t complete strictly in the order they appear; they are non-blocking and allow other operations to continue while waiting for results. Asynchronous programming is useful for tasks that can be slow, like I/O operations or network requests. You start a task, move on to the next, and handle the result later using callbacks, promises, or async/await.
As you’ve learned, asynchronous JavaScript allows tasks to run later without blocking the main thread. This is often used for operations like I/O, network requests, or timers, letting other code execute while waiting for results.
Code Example:
console.log("Start")
setTimeout(() => {
console.log("Async task done")
}, 2000)
console.log("End")
Here, console.log("End") runs before the setTimeout callback because setTimeout is non-blocking.
Note: The core ECMAScript language itself doesn’t provide asynchronous features. Functions like setTimeout, fetch, or fs.readFile come from the runtime environment, Web APIs in browsers and Libuv in Node.js. Without these, JavaScript executes all code synchronously, one line at a time.
The following examples will help you see how asynchronous behavior works:
When learning about asynchronous in JavaScript, it’s important to understand how non-blocking operations work. In Node.js, asynchronous methods allow the main thread to continue executing other tasks while waiting for I/O operations to complete.
This demonstrates the efficiency of synchronous and asynchronous in JavaScript for handling I/O-heavy tasks without blocking the main thread.
Code Implementation:
let fs = require("fs")
console.log("Started reading file asynchronously")
fs.readFile("./somefile.txt", "utf8", (error, data) => {
if (error) {
console.log(error)
} else {
console.log("File Content:", data)
}
})
console.log("Ended")
Code Walkthrough:
A JavaScript promise represents the future value of an asynchronous operation. It acts as a placeholder for a result that will be available later. A promise has three states: pending, fulfilled (success), and rejected (failure).
Promises are commonly used for asynchronous tasks that take time to complete, helping manage code in a non-blocking way.
Code Implementation:
let fs = require("fs")
function makePromise() {
return new Promise((resolve, reject) => {
fs.readFile("./promises.txt", "utf-8", (err, data) => {
if (err) {
reject(err)
} else {
resolve(data)
}
})
})
}
function onSuccess(data) {
console.log("Data successfully read:", data)
}
function onError(err) {
console.log("Following error found:", err)
}
makePromise().then(onSuccess).catch(onError)
Code Walkthrough:
The .then/.catch syntax used to handle the success (fulfilled) and failure (rejected) states of a promise can be converted into async/await with a try-catch for error handling. This approach is often preferred when you have multiple asynchronous operations, as it makes the code easier to read and maintain.
Using .then/.catch:
function mainReceiver() {
makePromise().then(onSuccess).catch(onError)
}
mainReceiver()
Using async/await::
async function mainReceiver() {
try {
let data = await makePromise()
console.log("data already logged")
} catch (error) {
console.log("Error already logged")
}
}
mainReceiver()
Code Walkthrough:
Dealing with synchronous and asynchronous behavior in JavaScript can be tricky. Different browsers may handle async operations like timers, fetch calls, or DOM updates inconsistently, making it hard to predict behavior. Nested callbacks or mixed sync/async patterns can also create complex flows that are difficult to debug and maintain.
A practical solution is to use a cloud-based testing platform. It provides consistent environments across browsers and OSes, allowing you to reliably test both synchronous and asynchronous behavior, while offering real-time logging and monitoring to visualize the order and timing of actions.
One such platform is LambdaTest, which allows you to test synchronous and asynchronous JavaScript behavior across a wide range of browsers and operating systems without worrying about local setup or infrastructure.
With LambdaTest, you can simulate user interactions like clicks, typing, and navigation to observe how both immediate (synchronous) and delayed (asynchronous) actions execute in real time.
LambdaTest is a GenAI-native test execution platform that allows you to perform manual and JavaScript automation testing at scale across 3000+ browsers and OS combinations.
For example, automate a polynomial evaluator by injecting data synchronously for instant results, asynchronously for delayed updates, while LambdaTest ensures that your synchronous and asynchronous JavaScript executes correctly across all targeted browsers, enabling precise automation and effective debugging in JavaScript.
In your scenarios, sync or async behavior depends on browser response and automated actions, not Selenium’s async/await.
You’ll automate two scenarios:
Project Prerequisites:
Project Structure Setup:
Once you are done with your project setup, in the root of your project folder, add the following files:
This test validates synchronous data injection in a polynomial evaluator. Inputs are processed instantly, and results are verified in real time across browsers with LambdaTest.
Test Scenario:
Code Implementation:
require("dotenv").config()
const { Builder, By } = require("selenium-webdriver")
let assert = require("node:assert")
const USERNAME = process.env.LT_USERNAME
const ACCESS_KEY = process.env.LT_ACCESS_KEY
browserName: "chrome",
browserVersion: "latest",
"LT:Options": {
platformName: "Windows 10",
build: "Sync Data Injection",
name: "LambdaTest Synchronous Test",
selenium_version: "4.0.0",
},
}
async function syncAutomate() {
const driver = await new Builder()
.usingServer(GRID_URL)
.withCapabilities(capabilities)
.build()
try {
await driver.get("https://stevepurpose.github.io/polynomial-Evaluate/")
let coEntry = await driver.findElement(By.id("toPush"))
let sendEntry = await driver.findElement(By.id("but1"))
let varEntry = await driver.findElement(By.id("toSub"))
let displayInfo = await driver.findElement(By.id("to_read"))
let takeSum = await driver.findElement(By.id("but3"))
let showEntry = await driver.findElement(By.id("but2"))
await coEntry.sendKeys(2); await sendEntry.click()
await coEntry.sendKeys(0); await sendEntry.click()
await coEntry.sendKeys(1); await sendEntry.click()
await coEntry.sendKeys(0); await sendEntry.click()
await coEntry.sendKeys(1); await sendEntry.click()
await varEntry.sendKeys(2); await takeSum.click()
let info = await displayInfo.getText()
console.log(info)
assert(info.includes("37"), "Expected result to contain 37")
await showEntry.click()
let info2 = await displayInfo.getText()
console.log(info2)
} finally {
await driver.quit()
}
}
syncAutomate()
Code Walkthrough:
In your console, you will see:
Asynchronous automation on LambdaTest involves testing behaviors that don’t happen immediately. they’re delayed, event-driven, or data-driven.
Test Scenario:
Code Implementation:
Using LambdaTest Tunnel to give LambdaTest access to your local webpage.
require("dotenv").config()
const { Builder, By } = require("selenium-webdriver")
const USERNAME = process.env.LT_USERNAME
const ACCESS_KEY = process.env.LT_ACCESS_KEY
const capabilities = {
browserName: "Chrome",
browserVersion: "latest",
platformName: "Windows 10",
"LT:Options": {
build: "Async Data Injection",
name: "Non-blocking SendKeys Logging",
selenium_version: "4.0.0",
tunnel: true,
},
}
async function asyncAutomate() {
const driver = await new Builder()
.usingServer(GRID_URL)
.withCapabilities(capabilities)
.build()
try {
await driver.get("[http://localhost:8080/index.html](http://localhost:8080/index.html)")
const syncPara = await driver.findElement(By.id("sync"))
const asyncPara = await driver.findElement(By.id("async"))
// Async injection after delay
setTimeout(async () => {
await asyncPara.sendKeys("Asynchronous injection after delay")
const asyncText = await asyncPara.getText()
console.log("Async Paragraph Text:", asyncText)
}, 5000)
// Synchronous injection runs immediately
await syncPara.sendKeys("Synchronous injection completed")
const syncText = await syncPara.getText()
console.log("Sync Paragraph Text:", syncText)
// Wait to capture async log
await driver.sleep(7000)
} finally {
await driver.quit()
}
}
asyncAutomate()
Code Walkthrough:
Run Setup:
Start your HTTP Server:
npm run server
Execute your Script:
node asyncauto.js
To get started, follow this support documentation on JavaScript with Selenium to run your first JavaScript test on LambdaTest.
Synchronous and Asynchronous in JavaScript define how tasks are executed in the runtime. Synchronous code runs sequentially, blocking the main thread until each task completes, while asynchronous code allows tasks to run independently, enabling non-blocking execution and better performance for I/O-heavy operations.
Measures | Synchronous | Asynchronous |
---|---|---|
Execution | Runs one operation at a time, line by line in order. | Runs independently; tasks may be completed out of order. |
Performance | Blocks the main thread; the new task waits for the previous task to finish. | Non-blocking; main thread continues while tasks are processed asynchronously. |
Rendering Responsiveness | Long-running tasks may freeze the UI in the browser. | UI remains responsive even during long-running tasks. |
Compatibility | Not suitable for modern needs like real-time apps, live updates, or API calls. | Well-suited for real-time apps, network requests, and other modern use cases. |
Use Cases | Good for simple, quick operations where blocking is not an issue. | Supports network requests, file I/O, timers, and other delayed operations. |
Complexity | Easier to write, read, and debug. | More complex; requires understanding of callbacks, promises, or async/await. |
Error Handling | Errors can be handled directly using try-catch. | Errors are handled via callbacks, .catch(), or try-catch in async/await. |
Debugging | Straightforward because tasks run sequentially. | It can be tricky due to out-of-order execution and event loop behavior. |
Real-world I/O Impact | Slower for file reads, network requests, or database calls because the thread is blocked. | Efficient for I/O-heavy operations, as tasks run in the background without blocking the main thread. |
Memory and Resource Management | Uses fewer resources for small, sequential tasks. | Better scalability for large operations, but may require attention to managing multiple asynchronous tasks simultaneously. |
Select synchronous JavaScript for straightforward, immediate tasks, and asynchronous for operations needing concurrency, external calls, or non-blocking execution.
When working with synchronous and asynchronous in JavaScript, certain mistakes often lead to bugs or unpredictable behavior.
Understanding these common pitfalls helps you write more reliable code.
Writing functions that behave synchronously under some conditions and asynchronously under others. For example, inconsistentRead calls the callback synchronously if the data exists in cache, but asynchronously if it doesn’t. This can confuse callers and cause unpredictable behavior.
Solution:
Make the function consistently asynchronous using process.nextTick().
let fs = require("fs")
const cache = new Map()
function consistentRead(filename, cb) {
if (cache.has(filename)) {
// Always call callback asynchronously
process.nextTick(() => cb(null, cache.get(filename)))
} else {
fs.readFile(filename, "utf8", (err, data) => {
if (err) return cb(err)
cache.set(filename, data)
cb(null, data)
})
}
}
Callbacks nested within each other (“pyramid of doom”) make code unreadable and hard to debug.
Solution:
Use Promises for readability.
function pMaker1() { return new Promise((resolve, reject) => { /*...*/ }) }
function pMaker2() { return new Promise((resolve, reject) => { /*...*/ }) }
function pMaker3() { return new Promise((resolve, reject) => { /*...*/ }) }
pMaker1().then(onSuccess1).then(onSuccess2).then(onSuccess3).catch(onReject)
Asynchronous operations fail (throw or reject) without a handler, causing unhandled errors.
Solution:
Handle errors in Promises using .then/.catch:
doSomething()
.then(result => console.log(result))
.catch(err => console.log(err))
Besides these, developers often encounter other common JavaScript errors that make debugging challenging.
Even with flawless front-end code, certain functionalities may fail, especially during cross-browser compatibility testing. Understanding these pitfalls helps prevent unexpected behavior and improve code reliability.
Certain considerations need to be made when we think of writing asynchronous code with JavaScript; you can see some of them below:
Understanding synchronous and asynchronous JavaScript is crucial for building efficient applications. Start with callbacks, master promises, and then use async/await for the most readable and maintainable code.
Author's Profile
Stephen Odogwu
Stephen Odogwu, alias Steve purposeis, is a fullstack software developer with 3+ years of experience in JavaScript, Node.js, React.js, Express.js, Python, and Jest. He has expertise in Django, FastAPI, Selenium WebDriver, Locust, AWS (EC2, S3, Lambda), GitHub Actions, and LambdaTest. Steve is currently building Stimutter, an open source, lightweight event driven state management library. He contributes to open source projects like Node.js and has submitted pull requests to documentation platforms such as Jest and MDN. In his personal projects, he uses test driven development with Jest and Selenium WebDriver, integrating the tests into GitHub Actions CI/CD pipelines.
Hubs: 01
Author's Profile
Stephen Odogwu
Stephen Odogwu, alias Steve purposeis, is a fullstack software developer with a B Eng in Civil Engineering and 3+ years of experience in JavaScript, Node.js, React.js, Express.js, Python, and Jest. He has also got expertise with Django, FastAPI, Selenium WebDriver, Locust, AWS (EC2, S3, Lambda), GitHub Actions, and LambdaTest. Steve is currently building Stimutter, an open source, lightweight event driven state management library. He contributes to open source projects like Node.js and has submitted pull requests to documentation platforms such as Jest and MDN. In his personal projects, he uses test driven development with Jest and Selenium WebDriver, integrating the tests into GitHub Actions CI/CD pipelines.
Hubs: 00
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