WebSocket wrapper breaking code that extends the WebSocket object

[yaief]

Environment

Everything

How to reproduce

Extend WebSocket in JS.
In this case extended through TypeScript, but obviously similar effect can be had by raw JS

Observed behaviour

WebSocket breaks on creation

Expected behaviour

WebSocket works

This is the adblock plus code:

function WrappedWebSocket(url)
    {
      // Throw correct exceptions if the constructor is used improperly.
      if (!(this instanceof WrappedWebSocket)) return RealWebSocket();
      if (arguments.length < 1) return new RealWebSocket();

Note that the line with the first if-statement has an error where it call return RealWebSocket instead of return new RealWebSocket

This is triggered by "this" being set to Window when extending WebSocket, and the crashing on "return RealWebSocket"

Notes

• Also see issue #4563

[kzar]

Please give a concrete example (ideally in plain JavaScript and as simple as possible) of how we can reproduce this problem.

Please give an example of an environment where this happens. For example a specific version of Chrome, OS and Adblock Plus.

[sebastian]

Replying to yaief:

function WrappedWebSocket(url)
    {
      // Throw correct exceptions if the constructor is used improperly.
      if (!(this instanceof WrappedWebSocket)) return RealWebSocket();
      if (arguments.length < 1) return new RealWebSocket();

Note that the line with the first if-statement has an error where it call return RealWebSocket instead of return new RealWebSocket

This is on purpose, so that when you call WebSocket() (without new keyword) it errors out in the same way as it would without the wrapper.

I also cannot think of a scenario, from the top of my head, where this would break any code that would work without the wrapper. Also this is a duplicate of #4563, so I'm closing this issue now. Feel free to reopen the original issue, once there is actually a way to reproduce this.