how can I create an advertiser with web Bluetooth API? now I am only able to scan nearby Bluetooth devices.
but how can I make my laptop a peripheral device that advertises data to another Bluetooth device to establish a connection? with(web BLE API)
The specification of Web Bluetooth is very clear about it:
The first version of this specification allows web pages, running on a UA in the Central role, to connect to GATT Servers over either a BR/EDR or LE connection.
The answer is: you can't.
There is a specification issue for Advertisement /Beacon Broadcast Support.
And, a Chromium Issue 1043409: Bluetooth Advertiser which you can Star to follow.