Promise的使用

mac2022-06-30 21

<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>Document</title> </head> <body> <script> let a = 1 let b = 2 let promise = new Promise(function(resolve, reject){ //规定就TM这么写 if(a==1) { resolve(b) } }) // 上面等于下面这个 // let promise2 = Promise.resolve(2) promise.then(success => console.log(success)).catch(err => console.log(err)) //2 //成功了就then, 失败就catch. //.then 里面可以接收一个或者两个函数，成功(resolve)了走第一个函数，失败(reject)了走第二个函数 promise.then(success => {console.log(success);return success}).then(success => console.log(success)) //2 2 let p2 = promise.then(success => {console.log(success)}) //2 console.log(p2) //Promise {<pending>} 这个会最先返回，因为上面的都是异步 p2.then(success => console.log(success)) //undefined 因为上一步并没有return任何值过来 let p3 = promise.then(success => {console.log(success);return success}) //2 console.log(p3) //Promise {<pending>} 第二个返回 console.log(promise) //Promise {<resolved>: 2} 第三个返回 //promise有三种状态：pending/reslove/reject 。pending就是未决，resolve可以理解为成功，reject可以理解为拒绝。 //Promise.all的使用。都成功了走then,失败了catch let a1 = Promise.resolve("aaa") let a2 = Promise.resolve("bbb") let a3 = Promise.resolve("ccc") let a4 = Promise.reject("ddd") Promise.all([a1,a2,a3]).then(res=>console.log(res)).catch(err=>console.log(err)) //["aaa","bbb","ccc"] Promise.all([a1,a2,a3,a4]).then(res=>console.log(res)).catch(err=>console.log(err)) //ddd </script> </body> </html>

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