# Async and await
# Async
Syntactical sugar for promises. Same as returning a new promise object in function.
- Async function always return a promise.
- If the function returns a value, the promise will be resolved with that value.
- If the function throws an exception, the promise will be rejected.
async function greet(){
return 'hi'
}
// This returns promise resolved with value 'hi'
greet().then((val) => {
console.log('resolved with:', val)
})
// resolved with: hi
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function add(x,y){
if (typeopf x !== 'number' || typeof y !== 'number'){
throw 'X and Y must be numbers!'
}
return x + y;
}
// this returns either a value or throws an error
async function add(x,y){
if (typeopf x !== 'number' || typeof y !== 'number'){
throw 'X and Y must be numbers!'
}
return x + y;
}
// this returns a promise object with either resolved or rejected value
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# Error handling
async function add(x,y){
if (typeopf x !== 'number' || typeof y !== 'number'){
throw 'X and Y must be numbers!'
}
return x + y;
}
add('e', 'r')
.then((val)=> {
console.log('resolved with', val)
})
.catch((err) => {
console.log('rejected with', err)
})
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# Await
- Can only be used inside function declared with async
- await will pause the function until the promise is resolved
- Used instead of
then.
async function getPlanets(){
const res = axios.get('https://swapi.co/api/planests/')
console.log(res.data)
}
// undefined
async function getPlanets(){
const res = await axios.get('https://swapi.co/api/planests/')
console.log(res.data)
}
// good!
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# Error handling
async function getPlanets(){
const res = await axios.get('https://swapi.co/api/planests/')
console.log(res.data)
}
// catch
getPlanets.catch((err)=>{
console.log(err)
})
// try catch
async function getPlanets(){
try {const res = await axios.get('https://swapi.co/api/planests/')
console.log(res.data)
} catch(err){
console.log('caught', err)
}
}
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# Parallel vs Sequential
