Understanding the Difference Between setTimeout and setInterval for API Calls in JavaScript

When making API calls at regular intervals in JavaScript, two common timing functions are often considered: setTimeout and setInterval. While both can be used to execute a function after a certain delay, they behave differently, which can significantly impact performance and reliability when making API requests.

How setInterval Works

setInterval is a built-in JavaScript function that repeatedly executes a given function at specified intervals, without waiting for the previous execution to complete.

setInterval(() => {
    fetch("https://api.example.com/data")
        .then(response => response.json())
        .then(data => console.log(data))
        .catch(error => console.error("Error fetching data:", error));
}, 5000); // Executes every 5 seconds

Issues with setInterval for API Calls

Overlapping Requests: If the API takes longer to respond than the interval duration, multiple requests may stack up, leading to race conditions or rate limiting issues.
No Error Handling for Delays: setInterval continues executing at fixed intervals even if the previous request hasn't finished yet.
No Control Over Next Execution: If an API request fails or takes too long, setInterval does not adjust dynamically.

How setTimeout Works

Unlike setInterval, setTimeout only executes a function once after a delay. However, by calling setTimeout recursively, you can create a controlled interval between API requests.

function fetchData() {
    fetch("https://api.example.com/data")
        .then(response => response.json())
        .then(data => console.log(data))
        .catch(error => console.error("Error fetching data:", error))
        .finally(() => {
            setTimeout(fetchData, 5000); // Waits 5 seconds before the next request
        });
}

fetchData(); // Start the loop

Advantages of Using setTimeout

Prevents Overlapping Requests: The next request is only scheduled after the previous one completes.
Handles Errors Gracefully: If an API request fails, you can implement a retry strategy before scheduling the next request.
More Control Over Execution: You can adjust the interval dynamically based on API response times, network conditions, or error handling logic.

When to Use Which?

Scenario	Use `setInterval`	Use `setTimeout`
Fetching real-time data (e.g., stock prices, sports scores) where missing occasional updates is acceptable.	✅	❌
Ensuring each API call finishes before making the next one to prevent overlapping requests.	❌	✅
Implementing dynamic delays based on response times or errors.	❌	✅
Running animations or UI updates at a fixed interval (not API-related).	✅	❌

Conclusion

For API polling, setTimeout is generally a better choice than setInterval because it ensures that each request completes before scheduling the next one. This prevents overlapping requests and allows better error handling. setInterval can be useful when precise timing is more important than waiting for a response, but in most cases, setTimeout provides better reliability and control.

By choosing the right approach, you can optimize API polling in JavaScript applications while avoiding unnecessary requests and performance issues.

Oussama GHAIEB