+ stricter validation for source and generated line/column: when they…

… exist, they MUST be numbers. Related to mozilla#385, though I choose to stick with the exception being thrown as that better ensures code/mapping quality AFAIAC.

+ `addMapping()`: ensure that `originalLine` and `originalColumn` are `null` when `original` argument was undefined/`null`.

lib/source-map-generator.js

@@ -124,8 +124,8 @@ class SourceMapGenerator {
     this._mappings.add({
       generatedLine: generated.line,
       generatedColumn: generated.column,
-      originalLine: original != null && original.line,
-      originalColumn: original != null && original.column,
+      originalLine: original && original.line,
+      originalColumn: original && original.column,
       source,
       name
     });
@@ -261,50 +261,66 @@ class SourceMapGenerator {
    * in to one of these categories.
    */
   _validateMapping(aGenerated, aOriginal, aSource, aName) {
-    // When aOriginal is truthy but has empty values for .line and .column,
-    // it is most likely a programmer error. In this case we throw a very
-    // specific error message to try to guide them the right way.
-    // For example: https://github.com/Polymer/polymer-bundler/pull/519
-    if (
-      aOriginal &&
-      typeof aOriginal.line !== "number" &&
-      typeof aOriginal.column !== "number"
-    ) {
-      throw new Error(
-        "original.line and original.column are not numbers -- you probably meant to omit " +
-          "the original mapping entirely and only map the generated position. If so, pass " +
-          "null for the original mapping instead of an object with empty or null values."
-      );
-    }
-
     if (
       aGenerated &&
-      "line" in aGenerated &&
-      "column" in aGenerated &&
+      typeof aGenerated.line === "number" &&
+      typeof aGenerated.column === "number" &&
       aGenerated.line > 0 &&
       aGenerated.column >= 0 &&
       !aOriginal &&
       !aSource &&
       !aName
     ) {
       // Case 1.
+      return;
     } else if (
       aGenerated &&
-      "line" in aGenerated &&
-      "column" in aGenerated &&
+      typeof aGenerated.line === "number" &&
+      typeof aGenerated.column === "number" &&
       aOriginal &&
-      "line" in aOriginal &&
-      "column" in aOriginal &&
+      typeof aOriginal.line === "number" &&
+      typeof aOriginal.column === "number" &&
       aGenerated.line > 0 &&
       aGenerated.column >= 0 &&
       aOriginal.line > 0 &&
       aOriginal.column >= 0 &&
       aSource
     ) {
       // Cases 2 and 3.
+      return;
     } else {
+      let errMsg;
+
+      // When `aOriginal` is truthy but has empty values for .line and .column,
+      // it is most likely a programmer error. In this case we throw a very
+      // specific error message to try to guide them the right way.
+      // For example: https://github.com/Polymer/polymer-bundler/pull/519
+      if (
+        aOriginal &&
+        (typeof aOriginal.line !== "number" ||
+        typeof aOriginal.column !== "number")
+      ) {
+        errMsg = 
+            "original.line and/or original.column are not numbers -- you probably meant to omit " +
+            "the original mapping entirely and only map the generated position. If so, pass " +
+            "null for the original mapping instead of an object with empty or null values.";
+      }
+
+      // When `aGenerated` is truthy but has empty values for .line and .column,
+      // it is most likely a programmer error. In this case we throw a very
+      // specific error message to try to guide them the right way.
+      else if (
+        aGenerated &&
+        (typeof aGenerated.line !== "number" ||
+        typeof aGenerated.column !== "number")
+      ) {
+        errMsg = 
+          "generated.line and/or generated.column are not numbers -- you MUST specify " +
+            "a valid generated position.";
+      }
+
       throw new Error(
-        "Invalid mapping: " +
+        (errMsg ? errMsg + " " : "") + "Invalid mapping: " +
           JSON.stringify({
             generated: aGenerated,
             source: aSource,