javascript - 如何检查获取调用并返回相同的调用

Question

我需要一种接受 promise 的方法，在其上调用 .then 来检查返回值，然后将这个 promise 完全按照系统其他部分的原样返回。上下文是我正在尝试从 GraseMonkey 脚本修改获取 API，以便我可以修改返回的数据。我通过在响应中调用 .json() 来检查它。但是，如果我不修改数据，我需要返回一个对象，该对象完全代表对获取 API 的原始调用，以便页面代码看不出有什么不同。但是当我尝试返回对象时，我得到一个错误，提示响应已经被消耗，现在我迷失在一堆我似乎无法解决的 promise 中(我不是 JS 本地人)

下面的代码是我已经拥有的并且它可以工作，但它并不是真正可以接受的，因为它重复了所有其他没有被破坏的请求。

function newFetch(){
    if (needsToBeModified(arguments[0])) {

        response = null;
        return oldFetch.apply(this, arguments)
            .then(r => {
                response = r;
                return r.json();
            })
            .then(
                j => {
                    j = processJSON(j);
                    return new Promise((resolve, rej) => {
                        response.json = () => new Promise((resolve, reject) => resolve(j));
                        resolve(response);
                    });
                },
                (fail) => {
                    return oldFetch.apply(this, arguments)
                    //How can I avoid making this call here again?
                }
            );
    } else {
        return oldFetch.apply(this, arguments);
    }      
}

谁能告诉我一种查看 json 的方法，如果这会引发错误，则无需再次调用即可从 fetch 返回原始 promise？

谢谢。

Answer 1

fetch() 返回解析为 response object 的 promise .该响应对象的方法之一是 .clone()这听起来就像你想要的那样。来自 .clone() 的文档:

The clone() method of the Response interface creates a clone of a response object, identical in every way, but stored in a different variable.

clone() throws a TypeError if the response Body has already been used. In fact, the main reason clone() exists is to allow multiple uses of Body objects (when they are one-use only.)

我想你可以这样使用它:

function newFetch(){
    let p = oldFetch.apply(this, arguments);
    if (needsToBeModified(arguments[0])) {
        let origResponse, cloneResponse;
        return p.then(r => {
            origResponse = r;
            cloneResponse = r.clone();
            return r.json();
        }).then(j => {
            j = processJSON(j);
            // monkey patch .json() so it is a function that resolves to our modified JSON
            origResponse.json = () => Promise.resolve(j);
            return origResponse;
        }, fail => {
            // return clone of original response
            if (cloneResponse) {
                return cloneResponse;
            } else {
                // promise was rejected earlier, don't have a clone
                // need to just propagate that rejection
                throw fail;
            }
        });
    } else {
        return p;
    }      
}