next.js/packages/next/src/client/remove-base-path.ts

import { hasBasePath } from './has-base-path'

const basePath = (process.env.__NEXT_ROUTER_BASEPATH as string) || ''

export function removeBasePath(path: string): string {
  if (process.env.__NEXT_MANUAL_CLIENT_BASE_PATH) {
    if (!hasBasePath(path)) {
      return path
    }
  }

  // Can't trim the basePath if it has zero length!
  if (basePath.length === 0) return path

  path = path.slice(basePath.length)
  if (!path.startsWith('/')) path = `/${path}`
  return path
}