declare can be used to determine if a variable or array has been defined. In this example, declare is used to determine if foo has been defined.

declare -p foo

If foo has not been defined, something like this should be returned.

bash: declare: foo: not found

If foo has been defined, something like this should be returned. Two dashes (--) means that foo is a variable, not an array.

declare -- foo="bar"

-a means that foo is an array, not a variable.

declare -a foo='([0]="bar")'