What are the asynchronous functions of JavaScript ES6
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Asynchronous functionality consists of two parts:
1) Regular async functions prefixed with function
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async function fetchdata(url){
// Do something
// Always returns a promise
}
2) await uses keywords before asynchronous function calls in the main Async function.
One example is worth a thousand words. The following is a rewrite of the Promise-based example to use the Async function:
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function getasync(url) { // same as original function
return new Promise((resolve, reject) => {
const xhr = new XMLHttpRequest()
xhr.open("GET", url)
xhr.onload = () => resolve(xhr.responseText)
xhr.onerror = () => reject(xhr.statusText)
xhr.send()
})
}
async function fetchdata(){ // main Async function
var text1 = await getasync('test.txt')
console.log(text1)
var text2 = await getasync('test2.txt')
console.log(text2)
var text3 = await getasync('test3.txt')
console.log(text3)
return "Finished"
}
fetchdata().then((msg) =>{
console.log(msg) // logs "finished"
})
The above example runs with "test.txt","test2.txt","test3.txt", and finally "done" in that order.
As you can see, inside asynchronous functions, we refer to the asynchronous function getasync() as a synchronous function-there is no need for then() methods or callback functions to indicate the next step. Whenever await encounters a keyword, execution pauses until getasync() resolves, and then moves to the next line in the Async function. The result is the same as a purely Promised based approach, using a series of then() methods.
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