Solution looks weird but works.

_012_addBinary.js

@@ -0,0 +1,56 @@
+/**
+ * @param {string} a
+ * @param {string} b
+ * @return {string}
+ */
+var addBinary = function(a, b) {
+    let aa = a;
+    let bb = b;
+    aLength = a.length;
+    bLength = b.length;
+    let pads = aLength - bLength;
+
+    if(aLength > bLength){
+        const pad = aLength - bLength;
+
+        for(let i = 0; i<pad; i++){
+            bb = "0"+bb;
+        }
+    }else if(bLength > aLength){
+        const pad = bLength - aLength;
+
+        for(let i = 0; i<pad; i++){
+            aa = "0"+aa;
+        }
+    }
+
+    aLength = aa.length;
+    let output = "";
+    let carry  = 0;
+
+    for(let i = aLength -1 ; i>=0; i--){
+        let [newCarry, remainder] = mapper(aa[i], bb[i], carry);
+        carry = newCarry;
+        output = `${remainder}${output}`
+    }
+
+    if (carry == 1) {
+        output = `${carry}${output}`;
+    }
+
+    return output;
+};
+
+function mapper(a,b,c){
+    const key = `${a}${b}${c}`;
+    switch (key) {
+        case '000': return [0, 0];
+        case '001': return [0, 1];
+        case '010': return [0, 1];
+        case '011': return [1, 0];
+        case '100': return [0, 1];
+        case '101': return [1, 0];
+        case '110': return [1, 0];
+        case '111': return [1, 1];
+    }
+}