onMessage()

Register one and only one listener, per messageId per context. That will be called upon sendMessage from other side.

Optionally, send a response to sender by returning any value or if async a Promise.

messageId

Required | string

Any string that both sides of your extension agree on. Could be get-flag-count or getFlagCount or GET_FLAG_COUNT, as long as it's same in sender's sendMessage call.

callback

Required | fn

A callback function webext-bridge should call when a message is received with same messageId. The callback function will be called with one argument, a message which has sender, data and timestamp as its properties.

Optionally, this callback can return a value or a Promise, resolved value will sent as reply to sender.

Read security note before using this.