async 函数
async 返回的是promise, await 后面应该是个promise,否则立即resolve
try catch 防止rejected
async function myFunction() {
try {
await somethingThatReturnsAPromise();
} catch (err) {
console.log(err);
}
}
// 另一种写法
async function myFunction() {
await somethingThatReturnsAPromise()
.catch(function (err) {
console.log(err);
};
}
并发与继发
async function asyncPrint(value,ms){
let a = new Promise((resolve)=>{setTimeout(resolve,ms)});
let b = new Promise((resolve)=>{setTimeout(resolve,ms)});
await a;console.log(value+1111)
await b;console.log(value+22222)
}
asyncPrint('dddd',3000)//并发执行
-----------------------------------------
async function asyncPrint(value,ms){
await new Promise((resolve)=>{setTimeout(resolve,ms)});
console.log(value+1111)
await new Promise((resolve)=>{setTimeout(resolve,ms)});
console.log(value+22222)
}
asyncPrint('dddd',3000)//继发执行
------------------------------------
function P(){
return new Promise((resolve)=>{setTimeout(()=>resolve('dddd'),2000)})
}
let docs = [P(),P(),P()]
async function bingfa1(db){
let result = await Promise.all(docs);
console.log(result)
}
//Promise.all 并发执行
------------------------------------
async function jifa(db){
let result = [];
for(var i of [0,1,2]){
let val = await P();
result.push(val)
}
console.log(result)
}
jifa()
//for of 继发执行推荐
------------------------------------
async function bingfa2(db){
let result = [];
docs.forEach(async function(){
let a = await P();
result.push(a)
console.log(result)
})
console.log(result)
}
bingfa2()
//forEach 并发执行（不推荐）
