From 661634b4a535d7fd0b0fb65389decefa61ddffca Mon Sep 17 00:00:00 2001
From: Kanchalai Tanglertsampan <yuisu@microsoft.com>
Date: Fri, 29 Jan 2016 18:20:05 -0800
Subject: [PATCH] Fix lastEncodedSourceMapSpan from being set to undefined

---
 src/compiler/sourcemap.ts | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/compiler/sourcemap.ts b/src/compiler/sourcemap.ts
index 8abf1432b0c..86bad38cb6e 100644
--- a/src/compiler/sourcemap.ts
+++ b/src/compiler/sourcemap.ts
@@ -16,6 +16,14 @@ namespace ts {
     }
 
     let nullSourceMapWriter: SourceMapWriter;
+    // Used for initialize lastEncodedSourceMapSpan and reset lastEncodedSourceMapSpan when updateLastEncodedAndRecordedSpans
+    const defaultLastEncodedSourceMapSpan: SourceMapSpan = {
+        emittedLine: 1,
+        emittedColumn: 1,
+        sourceLine: 1,
+        sourceColumn: 1,
+        sourceIndex: 0
+    };
 
     export function getNullSourceMapWriter(): SourceMapWriter {
         if (nullSourceMapWriter === undefined) {
@@ -79,13 +87,7 @@ namespace ts {
 
             // Last recorded and encoded spans
             lastRecordedSourceMapSpan = undefined;
-            lastEncodedSourceMapSpan = {
-                emittedLine: 1,
-                emittedColumn: 1,
-                sourceLine: 1,
-                sourceColumn: 1,
-                sourceIndex: 0
-            };
+            lastEncodedSourceMapSpan = defaultLastEncodedSourceMapSpan;
             lastEncodedNameIndex = 0;
 
             // Initialize source map data
@@ -159,10 +161,12 @@ namespace ts {
                 // Pop sourceMapDecodedMappings to remove last entry
                 sourceMapData.sourceMapDecodedMappings.pop();
 
-                // Change the last encoded source map
+                // Point the lastEncodedSourceMapSpace to the previous encoded sourceMapSpan
+                // If the list is empty which indicates that we are at the beginning of the file,
+                // we have to reset it to default value (same value when we first initialize sourceMapWriter)
                 lastEncodedSourceMapSpan = sourceMapData.sourceMapDecodedMappings.length ?
                     sourceMapData.sourceMapDecodedMappings[sourceMapData.sourceMapDecodedMappings.length - 1] :
-                    undefined;
+                    defaultLastEncodedSourceMapSpan;
 
                 // TODO: Update lastEncodedNameIndex 
                 // Since we dont support this any more, lets not worry about it right now.