A customer macro: a fixed value, or a callback invoked with no arguments right before the URL is built. null omits parameters that use the macro; undefined means "no value" and falls through to the built-ins.
null
undefined
A customer macro: a fixed value, or a callback invoked with no arguments right before the URL is built.
nullomits parameters that use the macro;undefinedmeans "no value" and falls through to the built-ins.