我想知道是否可以像这样动态创建一个异步函数:

new Function('await Promise.resolve()');

预期，前面的代码抛出:

Uncaught SyntaxError: await is only valid in async function

最佳答案

是的，您可以获得对非全局的引用 AsyncFunction动态创建异步函数的构造函数。

您可以像这样获得对 AsyncFunction 构造函数的引用:

const AsyncFunction = Object.getPrototypeOf(async function(){}).constructor;

AsyncFunction 构造函数的签名是:

AsyncFunction(arg0?, arg1?, ...args?, functionBody: string);

注意:AsyncFunction 构造器不是全局的(就像Function 一样)，获取对它的引用的唯一方法是通过async function(){} 实例的原型(prototype)，如上图和 MDN docs 中所示.

示例1. 无参数的简单函数:

    const AsyncFunction = Object.getPrototypeOf(async function(){}).constructor;
    
    const myAsync = new AsyncFunction(`return true`);
    
    const result = await myAsync(); // true
    
    myAsync().then((result) => {
        // result is true
    });

上面的例子等同于:

    const myAsync = async () => {
        return true;
    };

示例 2. 定义参数:

    const AsyncFunction = Object.getPrototypeOf(async function(){}).constructor;
    
    // Define two named arguments: inValue, delay
    const asyncFn = new AsyncFunction('inValue', 'delay', `return new Promise((resolve) => {
        setTimeout(() => {resolve(inValue)}, delay);
    });`);
    
    // resolves to 'hello' after 100ms
    const result = await asyncFn('hello', 100);
    
    // After 200ms the promise will be resolved with the value 'world'
    asyncFn('world', 200).then((result) => {
        console.log(result);
    });

上面的例子等同于:

    const asyncFn = async (inValue, delay) => {
        return new Promise((resolve) => {
            setTimeout(() => {resolve(inValue)}, delay);
        });
    };

另一个很酷的事情是参数允许使用默认值、解构和剩余参数。

    const AsyncFunction = Object.getPrototypeOf(async function(){}).constructor;
    
    // Define defaults for inValue and delay
    const asyncFn = new AsyncFunction('inValue = "Hello"', 'delay = 100', `return new Promise((resolve) => {
        setTimeout(() => {resolve(inValue)}, delay);
    });`);
    
    // resolves to 'hello' after 100ms (because we defined default parameters)
    const result = await asyncFn();
    
    // resolves to 'world' after 200ms
    const result = await asyncFn('world', 200);

示例 3. 高级函数构造函数

请注意，如果 delay 值来自不受信任的来源(例如用户输入)，那么构造这样的函数可能是不安全的。

    const AsyncFunction = Object.getPrototypeOf(async function(){}).constructor;
    
    function createAsync(delay){
        return new AsyncFunction('inValue', `return new Promise((resolve) => {
            setTimeout(() => {resolve(inValue)}, ${delay});
        });`);
    }
    
    const asyncFn = createAsync(100);
    const asyncFn2 = createAsync(200);
    
    // After 100ms this will resolve 'hello'
    await result1 = asyncFn('hello');
    
    // After 200ms this will resolve 'world'
    await result2 = asyncFn2('world');

这大致相当于:

    function createAsync(delay){
        return async (inValue) => {
            return new Promise((resolve) => {
                setTimeout(() => {resolve(inValue)}, delay);
            };
        };
    }
    
    const asyncFn = createAsync(100);
    
    // After 100ms this will resolve 'hello'
    await result1 = asyncFn('hello');
    
    // After 100ms this will resolve 'world'
    await result2 = asyncFn('world');

请记住，您可以在动态创建的异步函数中使用 await :)

请注意，与 Function 构造函数和 eval() 相同的安全注意事项适用于异步函数构造函数。

如果您使用从不受信任来源接收的内容构建新函数，那么您的脚本或应用程序可能容易受到攻击 注入(inject)攻击。

关于javascript - 动态创建异步函数，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/47945024/