const sleep = async (time) => {
? ? return new Promise(resolve => setTimeout(resolve, time * 1000))
}
const sayHello = async () => {
? ? await sleep(2)
? ? console.log('Hello there')
}
sayHello()
這是解釋：
使用setTimeout()which 是一種內置方法，可在指定的毫秒數后調用函數或計算表達式。 setTimeout()有兩個參數，第一個是回調函數，第二個是毫秒數。1 秒 = 1000 毫秒所以 2 秒 = 2000 毫秒等等
function promised (val) {
? ? // Create a new promise and resolve val after 2 seconds
? ? return new Promise(resolve => setTimeout(() => resolve(val), 2000)) //2000ms => 2 seconds
}
const createPromise = promised('wait for it...') // Pass in your message to promised function
createPromise
? ? .then(val => console.log(val))
? ? // Catch errors if any you don't need it here since we are always resolving the promise i just included it here so you know it's exist?
? ? .catch(err => console.log(err))?
你應該總是使用 .catch 除非你 100% 確定承諾總是會解決
例子：
function greeting(name) {
? ? return new Promise((resolve, reject) => setTimeout(() => {
? ? ? ??
? ? ? ? try {
? ? ? ? ? ??
? ? ? ? ? ? if (name.length <= 2) {
? ? ? ? ? ? ? ? throw new Error('Name must be more than two characters')
? ? ? ? ? ? }
? ? ? ? } catch (msg) {
? ? ? ? ? ? reject(msg)
? ? ? ? }?
? ? ? ? resolve(`Hello ${name}`)
? ? ? ??
? ? ? ??
? ? }, 2000))
}
greeting('ab')
? ? .then(res => console.log(res))
? ? .catch(err => console.log(err)) // Error: Name must be more than two characters
greeting('John')
? ? .then(res => console.log(res)) // Hello John
? ? .catch(err => console.log(err))
使用異步，等待：
const greetSomeone = async (name) => {
? ? try {
? ? ? ? // Note that you can't use await outside an async function
? ? ? ? const msg = await greeting(name)?
? ? ? ? console.log(msg)
? ? } catch (err) {
? ? ? ? console.log(err)
? ? }
}
greetSomeone('ac') // Error: Name must be more than two characters
greetSomeone('Michael') // Hello Michael
