直接上一個例子，拉走，打開控制檯，結合代碼，一目瞭然
<html lang="en">
<head>
<meta charset="utf-8">
</head>
<body>
<script>
function sum(a, b) {
const promise = new Promise((resolve, reject) => {
if (typeof a != 'number' || typeof b != 'number') {
// reject參數返回一個執行錯誤對象，用.catch調用
reject({
msg: 'a or b not is a number'
})
}
// resolve參數返回一個執行正確對象，用.then調用
resolve({
sum: a + b
})
})
return promise
}
sum(8, 2).then(res => {
console.log(res)
}).catch(err => {
console.log(err)
})
</script>
</body>
</html>