function pairElement(str){

var pairs = {A:'T', T: 'A', C: 'G', G: 'C'};

return str.split("").map(x => [x,pairs[x]]);

}

/*

function pairElement(str) {

var result = [];

for (var i = 0; i < str.length; i += 1) {

if (str[i] === "A") {

result.push(["A", "T"]);

} else if (str[i] === "T") {

result.push(["T", "A"]);

} else if (str[i] === "C") {

result.push(["C", "G"]);

} else if (str[i] === "G") {

result.push(["G", "C"]);

}

}

return result;

}

*/

pairElement("GCG"); // should return [["G", "C"], ["C","G"],["G", "C"]]

pairElement("ATCGA"); // should return [["A","T"],["T","A"],["C","G"],["G","C"],["A","T"]].

pairElement("TTGAG"); // should return [["T","A"],["T","A"],["G","C"],["A","T"],["G","C"]].

pairElement("CTCTA"); // should return [["C","G"],["T","A"],["C","G"],["T","A"],["A","T"]].