socket.io-client - transports options array modified from [string] to [object] by io constructor

[esellier]

Describe the bug

const options = {transports: ["websocket","polling"]};
const s = io(<URL>, options);

options object is modified, and transports is set to [ class WS, class XHR ]
Not really a big issue, but quite unexpected and an unusual behaviour. Especially when you need to serialize the options object after connection.

Expected behavior
options object should not be mutated.

[esellier]

socket.io/packages/socket.io-client/lib/index.ts

Line 39 in 74599a6

opts = opts || {};

should be
opts = Object.assign({}, opts)

[darrachequesne]

You are right, that is not the cleanest way to use the options...

For reference, the transports array is modified there:

socket.io/packages/engine.io-client/lib/socket.ts

Line 1176 in 74599a6

o.transports = (o.transports || ["polling", "websocket", "webtransport"])

[idirdev]

Opened a fix for this in #5471.

The root cause is in packages/engine.io-client/lib/socket.ts — the Socket constructor assigns o directly to the caller's reference (const o = typeof uri === "object" ? uri : opts), then replaces o.transports with mapped constructor classes, which mutates the original object.

The fix shallow-clones the options ({ ...uri } / { ...opts }) so modifications stay internal. Also added a regression test.