__createBinding performance issue on web browsers

[jramsley]

__createBinding, which is used in __exportStar, uses accessor descriptors to get exported modules. In our product, we see a 10x performance degradation referencing exported enums this way compared to using a direct access object. We've verified that by refactoring our enum exports to be explicitly defined and not *, this issue is not seen.

Likewise, this issue is not seen in v1.14.0 when __createBinding looks like

    __createBinding = function(o, m, k, k2) {
        if (k2 === undefined) k2 = k;
        o[k2] = m[k];
    };

current implementation:

    __createBinding = Object.create ? (function(o, m, k, k2) {
        if (k2 === undefined) k2 = k;
        Object.defineProperty(o, k2, { enumerable: true, get: function() { return m[k]; } });
    }) : (function(o, m, k, k2) {
        if (k2 === undefined) k2 = k;
        o[k2] = m[k];
    });

JSBench:
https://jsbench.me/yokwmk1mau/1

[jakebailey]

With #168 merged, is this issue fixed? @jramsley can you still reproduce this in the latest tslib?

[jramsley]

It helps, but it is still tangibly slower using descriptors than accessing the object directly. It is still a 2x performance degradation compared to overwriting tslib with

    __createBinding = function(o, m, k, k2) {
        if (k2 === undefined) k2 = k;
        o[k2] = m[k];
    };

[ExE-Boss]

@jramsley

    __createBinding = function(o, m, k, k2) {
        if (k2 === undefined) k2 = k;
        o[k2] = m[k];
    };

The issue with that is that if m[k] is later mutated (or is a getter), the mutation won’t propagate when accessing o[k2], like it does in the case of real ES modules.