技術標籤：前端面試

promise

promise的幾種狀態：

• pending:初始狀態
• fulfilled:操作成功
• rejected:操作失敗

當成功時，會呼叫resolve並且執行then裡的內容；當失敗時呼叫reject並且執行catch裡的內容。

function A(){
           let p = new Promise(function(resolve, reject){
               setTimeout(function(){
                   var num = Math.ceil(Math.random()*20); //生成1-10的隨機數
 

                   console.log('隨機數生成的值：',num)
                   if(num<=10){
                       resolve(num);
                   }
                   else{
                       reject('數字太於10了即將執行失敗回撥');
                   }
               }, 2000);
              })
              return p
          }
 

    
       A().then(
           function(data){
               console.log("resolved成功回撥");
               console.log('成功回撥接受的值：',data);
           }
       )
       .catch(function(reason, data){
           console.log('catch到rejected失敗回撥');
           console.log('catch失敗執行回撥丟擲失敗原因：',
 
reason);
       });
       console.log(A)

function getStr1() {
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            resolve('我');
        }, 1000);
    });
}
function getStr2() {
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            resolve('愛');
        }, 1000);
    });
}
function getStr3() {
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            resolve('米');
        }, 1000);
    });
}
function getStr4() {
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            resolve('飯');
        }, 1000);
    });
}
getStr1().then(function (data) {
    console.log(data);
    return getStr2();
}).then(function (data) {
    console.log(data);
    return getStr3();
}).then(function (data) {
    console.log(data);
    return getStr4();
}).then(function (data) {
    console.log(data);
})

promise all

then裡面會接收所有promise返回的結果，以一個數組的形式返回
必須三個全部成功，只要有一個失敗，一個結果也得不到

function A(){
	let p=new Promise(function(resolve,reject){
		setTimeout(
			function(){
				var num=7;
				console.log(num);
				if(num<10){resolve(num)}
				else{reject('過大')}
			}
		,2000)
	})
	return p;
}
function B(){
	let p=new Promise(function(resolve,reject){
		setTimeout(
			function(){
				var num=6;
				console.log(num);
				if(num<10){resolve(num)}
				else{reject('過大')}
			}
		,2000)
	})
	return p;
}
function C(){
	let p=new Promise(function(resolve,reject){
		setTimeout(
			function(){
				var num=5
				console.log(num);
				if(num<10){resolve(num)}
				else{reject('過大')}
			}
		,2000)
	})
	return p;
}
Promise.all([A(),B(),C()]).then(function(data){console.log(data)})
.catch(function(reason,data){console.log('失敗原因：',reason)})