await and async is the second promise syntax that permits to manipulate promise.
The async/await syntax is part of ECMAScript 2017 and reduces the callback nesting find in the chaining syntax.
addOne = async function(x){
return x+1;
};
// Function declared as async so await can be used
(async () => {
// A aysnc function call returns a promise
promise = addOne(0)
//
let a;
try {
// You unwrapp the promise with the 'await' keyword
// You get the value (or an error is thrown)
a = await promise;
} catch (e) {
// You catch the error
console.log("Error! "+e);
}
//
// Tip And you can have the creation of the promise and the await call on the same line
a = await addOne(a);
console.log("The value of a is now "+a);
})();
async/await is a promises syntax handling where:
The async / await syntax reduce the callback nesting.
The return value of an async function is always a promise.
async function fn(x){
if (x) {
throw new Error("Error !");
} else {
return "All good";
}
};
// The return value of an ''async'' function is always a promise.
v = fn();
console.log("We go back a "+v.constructor);
addOne = async function(x){
return x + 1;
};
(async () => {
const a = await addOne(0);
console.log("The value after await is "+a);
})();
With a try/catch block.
failBad = async function(){
throw new Error("Bad failure")
};
(async () => {
try {
const a = await failBad(0);
console.log("The value after await is "+a);
} catch (e) {
console.log("Failure !!! with the message ("+e.message+")");
}
})();
// This function returns a fetch that returns a promise
// Don't return any other type of value otherwise the wait will have no effect
function wait(sec,name) {
if (typeof sec === undefined){
sec = 1;
}
if (typeof name === undefined){
name = 'Default Promise Name';
}
log(name+": Waiting "+sec+" seconds");
return fetch("https://gerardnico.com/wait.php?sleep="+sec)
.then(function(response) {
status = response.status;
log(name+": has waited "+sec+" seconds and returned (Statuscode: "+response.status+")");
return status;
})
}
function log(txt){
now = new Date(Date.now());
console.log(now.getHours()+":"+now.getMinutes()+":"+now.getSeconds()+" - "+txt);
}
( async () => {
log("Main: Creating promises");
promise = wait(5, "Promise");
log("Main: Await promise");
let returnValue = await promise;
log("Main: We got a return value ("+returnValue+")");
}
)();
// This function returns a fetch that returns a promise
// Don't return any other type of value otherwise the wait will have no effect
function wait(sec,name) {
if (typeof sec === undefined){
sec = 1;
}
if (typeof name === undefined){
name = 'Default Promise Name';
}
log(name+": Waiting "+sec+" seconds");
return fetch("https://gerardnico.com/wait.php?sleep="+sec)
.then(function(response) {
status = response.status;
log(name+": has waited "+sec+" seconds and returned (Statuscode: "+response.status+")");
return status;
})
}
function log(txt){
now = new Date(Date.now());
console.log(now.getHours()+":"+now.getMinutes()+":"+now.getSeconds()+" - "+txt);
}
log("Main: Creating promises");
let promises = [];
promises.push(wait(1, "Promise 1"));
promises.push(wait(5, "Promise 5"));
log("Main: Promise all promise");
// Promise create another thread and let the script execution continue
Promise
.all(promises)
.then(results=> log("PromiseAll: All promises has returned with "+results.length+" results"));
log("Main: End of the script will be fired immediately !");