Reference http://www.espruino.com/Reference#l_Wifi_connect mentions that options will have a way to pass expected ip address to use rather than dhcp... but as said: will - as future enhancement.
On the other hand, your access point may have the option to not do dhcp at all or for not for particular MAC addresses and define the repsective ip to use. May be you can control it there.
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Reference http://www.espruino.com/Reference#l_Wifi_connect mentions that options will have a way to pass expected ip address to use rather than dhcp... but as said: will - as future enhancement.
On the other hand, your access point may have the option to not do dhcp at all or for not for particular MAC addresses and define the repsective ip to use. May be you can control it there.