:nnoremap J gJ

Hello
    world

Hello    world

" Like gJ, but always remove spaces
fun! JoinSpaceless()
    execute 'normal gJ'

    " Character under cursor is whitespace?
    if matchstr(getline('.'), '%' . col('.') . 'c.') =~ 's'
        " When remove it!
        execute 'normal dw'
    endif
endfun

" Map it to a key
nnoremap <Leader>J :call JoinSpaceless()<CR>

xxxx=
123

xxxx123