Promise.prototype.catch（）没有捕获异常

Question

在下面的代码中，如果我在ajax函数的最开头插入行throw "Error";，则catch捕获异常并调用dispatchError。但是，如果它在xhttp.onreadystatechange箭头函数中抛出，则不会捕获异常。为什么？

   let ajax = (url, cb, data) => {

    let xhttp = new XMLHttpRequest();

    xhttp.onreadystatechange = () => {
        if (xhttp.readyState == 4) {
            let data;
            if (xhttp.responseText) {
                data = JSON.parse(xhttp.responseText);
            }
            cb({status: xhttp.status, data});
        }
    };

    xhttp.open(data?"POST":"GET", url, true);

    if (data) {
        xhttp.setRequestHeader("Content-Type", "application/json");
        xhttp.send(JSON.stringify(data));
    } else {
        xhttp.send();
    }
};

export const getData = () => {
    return new Promise((resolve, reject) => {
        ajax("url",response => response.status===200?
            resolve(response.data):
            reject(new Error("Error"))
        );
    });


getData()
    .then(d=>dispatchData(d))
    .catch(e=>dispatchError(e))