八、ES6中async函数

语法结构

异步函数声明式

1	async function name([param]){}

异步函数表达式

1	let name = async function([param]){}

用法示例：

async function myAsync() {
  return "hello world";
}
let promise = myAsync();
promise.then((value) => {
  console.log(value); // hello world
});

await表达式

await表达式用于等待一个Promise对象，它只能在异步函数中使用。

1	[return_value] = await expression;

return_value
返回Promise对象的处理结果。如果等待的不是Promise对象，则返回该值本身。
expression
一个Promise对象或者任何要等待的值

function createPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("执行成功了！");
    }, 200);
  });
}

async function myAsync() {
  console.log("当前异步函数");
  var result = await createPromise();
  console.log(result);
}

myAsync(); //

通过await处理报错。

function createPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject("执行成功了！");
    }, 200);
  });
}

async function myAsync() {
  console.log("当前异步函数");
  try {
    var result = await createPromise();
  } catch (e) {
    console.log("出错了", e);
  }
  console.log(result);
}

myAsync();

处理多个promise

let promise1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("one");
  }, 300);
});
let promise2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("two");
  }, 200);
});
let promise3 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("three");
  }, 100);
});
async function myAsync() {
  let result1 = await promise1;
  console.log(result1);

  let result2 = await promise2;
  console.log(result2);

  let result3 = await promise3;
  console.log(result3);
}
myAsync(); // one two three