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【JavaScript 】JavaScript 全教程 02

news 2025/11/29 13:32:17

.then() 方法 const promise = new Promise((resolve, reject) => {    
  setTimeout(() => {
    resolve('Result');
  }, 200);
});

promise.then((res) => {
  console.log(res);
}, (err) => {
  console.error(err);
});
 
.catch() 方法 const promise = new Promise(
  (resolve, reject) => {  
  setTimeout(() => {
    reject(Error('Promise 无条件拒绝。'));
  }, 1000);
});

promise.then((res) => {
  console.log(value);
});

promise.catch((err) => {
  console.error(err);
});
 
Promise.all() const promise1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(3);
  }, 300);
});
const promise2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(2);
  }, 200);
});
Promise.all([promise1, promise2]).then((res) => {
  console.log(res[0]);
  console.log(res[1]);
});
 
链接多个 .then() const promise = new Promise(
  resolve => 
    setTimeout(() => resolve('dAlan'),100)
);

promise.then(res => {
  return res === 'Alan' 
    ? Promise.resolve('Hey Alan!')
    : Promise.reject('Who are you?')
})
.then((res) => {
  console.log(res)
}, (err) => {
  console.error(err)
});
 
避免嵌套的 Promise 和 .then() const promise = new Promise((resolve, reject) => {  
  setTimeout(() => {
    resolve('*');
  }, 1000);
});
const twoStars = (star) => {  
  return (star + star);
};
const oneDot = (star) => {  
  return (star + '.');
};
const print = (val) => {
  console.log(val);
};
// 将它们链接在一起
promise.then(twoStars).then(oneDot).then(print);
 
JavaScript Async-Await 
异步 function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}

// 异步函数表达式
const msg = async function() {
  const msg = await helloWorld();
  console.log('Message:', msg);
}

// 异步箭头函数
const msg1 = async () => {
  const msg = await helloWorld();
  console.log('Message:', msg);
}

msg(); // Message: Hello World! <-- 2 秒后
msg1(); // Message: Hello World! <-- 2 秒后
 
解决 Promises let pro1 = Promise.resolve(5);
let pro2 = 44;
let pro3 = new Promise(function(resolve, reject) {
  setTimeout(resolve, 100, 'foo');
});
Promise.all([pro1, pro2, pro3]).then(function(values) {
  console.log(values);
});
// expected => Array [5, 44, "foo"]
 
异步等待 Promises function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}
async function msg() {
  const msg = await helloWorld();
  console.log('Message:', msg);
}
msg(); // Message: Hello World! <-- 2 秒后
 
错误处理 // 数据不完整
let json = '{ "age": 30 }';

try {
  let user = JSON.parse(json); // <-- 没有错误
  console.log( user.name );    // no name!
} catch (e) {
  console.error( "Invalid JSON data!" );
}
 
异步等待运算符 function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}
async function msg() {
  const msg = await helloWorld();
  console.log('Message:', msg);
}
msg(); // Message: Hello World! <-- 2 秒后
 