Understanding Asynchronous Calls
When it comes to dealing with asynchronous calls like making HTTP get requests using methods such as fetch, axios, AJAX, or working with Promises, it's crucial to understand that the return value at the end of the function may not be what you anticipate.
The Challenge with Asynchronous Calls
Asynchronous calls do not wait for the preceding call to finish execution before moving on to the next line of code. This can lead to unexpected outcomes, as demonstrated in the example below:
Even though we are demonstrating with fetch, the same behavior applies to other types of asynchronous calls.
getDataWithFetch();
function getDataWithFetch() {
console.log('start of outer function');
const url = 'https://jsonplaceholder.typicode.com/todos/1';
fetch(url).then(response => {
console.log('start of async function');
response.json();
console.log('end of async function');
});
console.log('end of outer function');
}
In the provided code snippet, the outer function completes before the async function kicks off, resulting in a return of undefined from getDataWithFetch(). Simply adding a return statement does not fully resolve this issue.
const response = getDataWithFetch();
console.log(response);
function getDataWithFetch() {
const url = 'https://jsonplaceholder.typicode.com/todos/1';
fetch(url).then(response => {
return response.json();
});
return 'hello world';
}
Even when returning the Promise object from fetch, we still do not obtain the desired response. To address this, we must make use of the then method to access the resolved data.
Working Effectively with Promises
Utilizing the "then" Method
By employing the then method, we can properly handle and retrieve the response data from our asynchronous calls:
getDataWithFetch().then(response => {
console.log(response);
});
function getDataWithFetch() {
const url = 'https://jsonplaceholder.typicode.com/todos/1';
return fetch(url).then(response => response.json());
}
This approach enables us to efficiently fetch and process the response data.
Implementing Async and Await
Recent Javascript features such as async and await offer a more concise way to manage asynchronous tasks. Here is an illustration utilizing async and await:
async function getDataWithFetch() {
const url = 'https://jsonplaceholder.typicode.com/todos/1';
const response = await fetch(url);
return response.json();
}
getDataWithFetch().then(response => {
console.log(response);
});
async and await simplify interaction with Promises, making the code flow appear synchronous even though it operates asynchronously.