How can I check if a program exists from a Bash script?

Question

How would I validate that a program exists, in a way that will either return an error and exit, or continue with the script?

It seems like it should be easy, but it\

Answer 1

hash foo 2>/dev/null: works with Z shell (Zsh), Bash, Dash and ash.

type -p foo: it appears to work with Z shell, Bash and ash (BusyBox), but not Dash (it interprets -p as an argument).

command -v foo: works with Z shell, Bash, Dash, but not ash (BusyBox) (-ash: command: not found).

Also note that builtin is not available with ash and Dash.