"last" with cancelation

last-with-cancel.js

"use strict";

var pending = { then: function () {} };

// operation must accept as its first argument a cancel token
// if your operation does not support cancelation, use a shim:
// last((_, ...args) => realOp(...args));
module.exports = operation => {
    let latestPromise = null;
    let previousCancel = null;

    return function () {
        if (previousCancel) {
            previousCancel();
        }

        const cancelToken = new CancelToken(c => previousCancel = c);
        const promiseForResult = operation.call(this, cancelToken, ...arguments);
        latestPromise = promiseForResult;

        return promiseForResult.then(
            value => {
                if (latestPromise === promiseForResult) {
                    return value;
                } else {
                    cancel throw;
                }
            },
            reason => {
                if (latestPromise === promiseForResult) {
                    throw reason;
                } else {
                    cancel throw;
                }
            }
        );
    };
};