This content originally appeared on DEV Community and was authored by DEV Community
Handling asynchronous became way easier thanks to the async/await pattern. It makes async tasks almost as straightforward as regular synchronous operations:
async function asyncCheckPokemon() {
const response = await fetch("https://pokeapi.co/api/v2/pokemon/25");
const pikachuJSON = await response.json();
// Prints "Look mum, no callbacks! Pokémon #25 is pikachu"
console.log(
`Look mum, no callbacks! ` +
`Pokémon #${pikachuJSON.id} is ${pikachuJSON.name}`
);
}
asyncCheckPokemon();
However, there are still some gotchas you should take into account when you have to deal with this pattern. The function in the previous example doesn't return anything, but it logs some text into the console.
Now, let's make a small change so that the function returns the string instead of printing it:
async function asyncCheckPokemon() {
const response = await fetch("https://pokeapi.co/api/v2/pokemon/25");
const pikachuJSON = await response.json();
return (
`Look mum, no callbacks! ` +
`Pokémon #${pikachuJSON.id} is ${pikachuJSON.name}`
);
}
const pikachuMessage = asyncCheckPokemon();
What's the value of the variable pikachuMessage? If you said that it's the string "Look mum, no callbacks! Pokémon #25 is pikachu", that's wrong. The variable is a promise that resolves to that string. But, why?
The async/await pattern works like this: When you execute an asynchronous task using the await keyword, the function that contains the call should be "paused" until the asynchronous task is over.
The thing is, you can't block the entire execution of your script because it also would block the event loop, the task wouldn't really be asynchronous anymore.
This problem is solved by using promises. If the function that contains the asynchronous task returns a promise, the asynchronous operation will block just that function.
Now, how do you handle async functions that return values? You should call it from another async function, and you should use await to make the call. It should look like this:
async function asyncCheckPokemon() {
const response = await fetch("https://pokeapi.co/api/v2/pokemon/25");
const pikachuJSON = await response.json();
return (
`Look mum, no callbacks! ` +
`Pokémon #${pikachuJSON.id} is ${pikachuJSON.name}`
);
}
async function printPikachuMessage() {
const pikachuMessage = await asyncCheckPokemon();
// Prints "Look mum, no callbacks! Pokémon #25 is pikachu"
console.log(pikachuMessage);
}
printPikachuMessage();
But, what if you want to call get the value from an async function at the top level (i.e. not inside a function)? In Node.js and Chrome, you can if the script is an ES6 module. In Node.js, make sure that the file extension is ".mjs" and for Chrome, add the type="module" attribute to the <script> tag.
But if you can't use modules, you'll have to use an async function that doesn't return anything, and then call that function at the top level. You can use an Immediately-Invoked Function Expression (IIFE) for this.
p.s. Did you like what you've read? Every week I send an email with free tips and insights to become a better JavaScript dev. If you're interested, click here to subscribe.
This content originally appeared on DEV Community and was authored by DEV Community
DEV Community | Sciencx (2021-05-20T15:35:59+00:00) I’m Using async/await. Why Does My Function Return a Promise?. Retrieved from https://www.scien.cx/2021/05/20/im-using-async-await-why-does-my-function-return-a-promise/
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