diff --git a/parse.js b/parse.js
index 38934aa..25dbf27 100644
--- a/parse.js
+++ b/parse.js
@@ -12,6 +12,7 @@
 {
     // break the input down into "parts": either words, digit strings, single character operators, or whitespace
     let parts = [];
+    let index = 0;
     while (input.length > 0)
     {
         // match our possible sequences of more than one character (words, digits, or whitespace)
@@ -20,14 +21,16 @@
             (match = input.match(/^\s+/)))
         {
             const matched = match[0];
-            parts.push(matched);
+            parts.push({index:index, part:matched});
             input = input.substr(matched.length);
+            index += matched.length;
         }
         else
         {
             // everything else is a single character (punctuation)
-            parts.push(input[0]);
+            parts.push({index:index, part:input[0]});
             input = input.substr(1);
+            index++;
         }
     }
 
@@ -36,7 +39,7 @@
     for (let i = 0; i < parts.length; i++)
     {
         const partsLeft = parts.length - i - 1;
-        let token = parts[i];
+        let { token, index } = parts[i];
         
         if (token.trim().length == 0)
         {
@@ -51,7 +54,7 @@
             {
                 token += "." + parts[i + 2];
             }
-            tokens.push({type:TokenType.NumberLit, token:token});
+            tokens.push({type:TokenType.NumberLit, token:token, index:index});
         }       
         else if (token == '"')
         {
@@ -99,17 +102,17 @@
                 throw "Unclosed string";
             }
             
-            tokens.push({type:TokenType.StringLit, token:token});
+            tokens.push({type:TokenType.StringLit, token:token, index:index});
         }
         else if (token.match(/\w+/))
         {
             // any keyword or identifier
-            tokens.push({type:TokenType.Word, token:token});
+            tokens.push({type:TokenType.Word, token:token, index:index});
         }
         else
         {
             // anything left is an operator
-            tokens.push({type:TokenType.Op, token:token});
+            tokens.push({type:TokenType.Op, token:token, index:index});
         }
     }