fix: if LEAST_UPPER_BOUND returns null, try GREATEST_LOWER_BOUND (#375)

packages/istanbul-lib-source-maps/lib/get-mapping.js

@@ -5,6 +5,10 @@
 'use strict';
 
 const pathutils = require('./pathutils');
+const {
+    GREATEST_LOWER_BOUND,
+    LEAST_UPPER_BOUND
+} = require('source-map').SourceMapConsumer;
 
 /**
  * AST ranges are inclusive for start positions and exclusive for end positions.
@@ -30,10 +34,11 @@ function originalEndPositionFor(sourceMap, generatedEnd) {
     // generated file end location. Note however that this position on its
     // own is not useful because it is the position of the _start_ of the range
     // on the original file, and we want the _end_ of the range.
-    const beforeEndMapping = sourceMap.originalPositionFor({
-        line: generatedEnd.line,
-        column: generatedEnd.column - 1
-    });
+    const beforeEndMapping = originalPositionTryBoth(
+        sourceMap,
+        generatedEnd.line,
+        generatedEnd.column - 1
+    );
     if (beforeEndMapping.source === null) {
         return null;
     }
@@ -47,7 +52,7 @@ function originalEndPositionFor(sourceMap, generatedEnd) {
         source: beforeEndMapping.source,
         line: beforeEndMapping.line,
         column: beforeEndMapping.column + 1,
-        bias: 2
+        bias: LEAST_UPPER_BOUND
     });
     if (
         // If this is null, it means that we've hit the end of the file,
@@ -70,6 +75,28 @@ function originalEndPositionFor(sourceMap, generatedEnd) {
     return sourceMap.originalPositionFor(afterEndMapping);
 }
 
+/**
+ * Attempts to determine the original source position, first
+ * returning the closest element to the left (GREATEST_LOWER_BOUND),
+ * and next returning the closest element to the right (LEAST_UPPER_BOUND).
+ */
+function originalPositionTryBoth(sourceMap, line, column) {
+    const mapping = sourceMap.originalPositionFor({
+        line,
+        column,
+        bias: GREATEST_LOWER_BOUND
+    });
+    if (mapping.source === null) {
+        return sourceMap.originalPositionFor({
+            line,
+            column,
+            bias: LEAST_UPPER_BOUND
+        });
+    } else {
+        return mapping;
+    }
+}
+
 function isInvalidPosition(pos) {
     return (
         !pos ||
@@ -98,7 +125,11 @@ function getMapping(sourceMap, generatedLocation, origFile) {
         return null;
     }
 
-    const start = sourceMap.originalPositionFor(generatedLocation.start);
+    const start = originalPositionTryBoth(
+        sourceMap,
+        generatedLocation.start.line,
+        generatedLocation.start.column
+    );
     let end = originalEndPositionFor(sourceMap, generatedLocation.end);
 
     /* istanbul ignore if: edge case too hard to test for */
@@ -128,7 +159,7 @@ function getMapping(sourceMap, generatedLocation, origFile) {
         end = sourceMap.originalPositionFor({
             line: generatedLocation.end.line,
             column: generatedLocation.end.column,
-            bias: 2
+            bias: LEAST_UPPER_BOUND
         });
         end.column -= 1;
     }