async/await实现Promise.acll()简介

        let a = new Promise((res, rej) => {
			res(1)
		}).catch(err => console.log(err))
		let b = new Promise((res, rej) => {
			setTimeout(() => {
				rej(2)
			}, 2000)
		}).catch(err => console.log(err))
		let c = new Promise((res, rej) => {
			res(3)
		}).catch(err => console.log(err))
       const arr = [a, b, c]

		async function bb() {
			let arr1 = [];
			try {
				for (let i = 0; i < arr.length; i++) {
					let h = await arr[i]
					arr1.push(h)
				}
			} catch (err) {
			}
			return arr1
		}
		
		bb().then(res => {
			console.log(res); //[1, undefined, 3]
		});

      const all = (arr) => {
  			return new Promise((resolve, reject) => {
				let length = arr && arr.length
				let count = 0
				let result = []
				if (!arr || arr.length === 0) {
					resolve(result)
				}
				arr.forEach(async (item, index) => {
					try {
						const res = await item
						result[index] = res
						count++
						if (count === length ) {
							resolve(result)
						}
					} catch (err) {
						reject(err)
					}
				});
			})
		}

         function fn() {
			return new Promise((resolve, reject) => {
				resolve(Math.random())
			})
		}
		async function asyncFunc() {
			let result
			try {
				result = await Promise.all([fn(), fn()])
				console.log(result)
			} catch (err) {
				console.log(err, 'err')
			}
			return result
		}
      asyncFunc().then(res => { console.log(res, 'res') })