I understood how to get deviceId in a proper way. It was not clear from documentation. It can be returned in this way:

amplitude.init(API_KEY).promise.then(function() {
  console.log(amplitude.getDeviceId())
})

Hi @jarren.patao .

Thank you for the answer. Just want to add my another mistake I did. I have two modules:

module A:

import * as amplitude from '@amplitude/analytics-browser'

...

amplitude.init(....)

module B:

import * as amplitude from '@amplitude/analytics-browser'

export const someHandler = () => {
  console.log(amplitude.getDeviceId()) // returns undefined
}

The mistake is importing amplitude twice as it creates second instance of amplitude. Instead of this I just export created instance from module A and use it in module B now:

 module A:

import * as amplitude from '@amplitude/analytics-browser'

export const amplitudeInstance = amplitude

...

amplitude.init(....)

module B:

import { amplitudeInstance } from 'moduleA'

export const someHandler = () => {
  console.log(amplitudeInstance.getDeviceId()) // it works ok
}

Solved :)