5.list.pl

% find the membership of X in the array.

member(X,[X|_]). % if X is head of the list that mean X is in this array, return true.
member(X,[_|T]):-member(X,T). % if X is not the head that means it can be in the next index of the array.
 % So, we do recursive by call member(X,T) which is the first statement again.

/* compare to JavaScript
function isValueExist(value, arr) {
 let X = value;
 let array = arr;
 
 let i;
 for (i = 0; i < array.length; i++) {
 if ( X === array[i]) return true;
 }
 
 return false;
} 

console.log(isValueExist(2, [1,2,3])); // return true

*/