Solution

caesar/caesar.js

@@ -1,5 +1,20 @@
-const caesar = function() {
+const caesar = function(string, shift) {
+    return string
+    .split("")
+    .map(char => shiftChar(char, shift))
+    .join("");
+};
 
-}
 
+let codeSets = code => (code < 97 ? 65 : 97);
+let mode = (m, n) => (m % n + n) % n;
+
+let shiftChar =(char, shift) => {
+    let code = char.charCodeAt();
+    
+    if((code  >= 65 && code <= 90) || (code >= 97 && code <= 122)){
+        return String.fromCharCode(mode(code + shift - codeSets(code), 26) + codeSets(code));
+    }
+    return char;
+};
 module.exports = caesar

caesar/caesar.spec.js

@@ -4,22 +4,22 @@ describe('caesar', function() {
   it('works with single letters', function() {
     expect(caesar('A', 1)).toEqual('B');
   });
-  xit('works with words', function() {
+  it('works with words', function() {
     expect(caesar('Aaa', 1)).toEqual('Bbb');
   });
-  xit('works with phrases', function() {
+  it('works with phrases', function() {
     expect(caesar('Hello, World!', 5)).toEqual('Mjqqt, Btwqi!');
   });
-  xit('works with negative shift', function() {
+  it('works with negative shift', function() {
     expect(caesar('Mjqqt, Btwqi!', -5)).toEqual('Hello, World!');
   });
-  xit('wraps', function() {
+  it('wraps', function() {
     expect(caesar('Z', 1)).toEqual('A');
   });
-  xit('works with large shift factors', function() {
+  it('works with large shift factors', function() {
     expect(caesar('Hello, World!', 75)).toEqual('Ebiil, Tloia!');
   });
-  xit('works with large negative shift factors', function() {
+  it('works with large negative shift factors', function() {
     expect(caesar('Hello, World!', -29)).toEqual('Ebiil, Tloia!');
   });
 });