fix: handle last empty line correctly when using update/overwrite (#274)

src/utils/Mappings.js

@@ -12,9 +12,12 @@ export default class Mappings {
 
 	addEdit(sourceIndex, content, loc, nameIndex) {
 		if (content.length) {
+			const contentLengthMinusOne = content.length - 1;
 			let contentLineEnd = content.indexOf('\n', 0);
 			let previousContentLineEnd = -1;
-			while (contentLineEnd >= 0) {
+			// Loop through each line in the content and add a segment, but stop if the last line is empty,
+			// else code afterwards would fill one line too many
+			while (contentLineEnd >= 0 && contentLengthMinusOne > contentLineEnd) {
 				const segment = [this.generatedCodeColumn, sourceIndex, loc.line, loc.column];
 				if (nameIndex >= 0) {
 					segment.push(nameIndex);

test/MagicString.js

@@ -491,6 +491,43 @@ describe('MagicString', () => {
 			assert.equal(loc4.line, 1);
 			assert.equal(loc4.column, 4);
 		});
+
+		it('generates a correct source map with update using content ending with a new line', () => {
+			const s = new MagicString('foobar');
+			s.update(2, 3, 'od\n');
+			s.update(4, 5, 'a\nnd\n');
+			assert.equal(s.toString(), 'food\nba\nnd\nr');
+
+			const map = s.generateMap({ hires: true });
+			const smc = new SourceMapConsumer(map);
+
+			// od\n
+			const loc = smc.originalPositionFor({ line: 1, column: 3 });
+			assert.equal(loc.line, 1);
+			assert.equal(loc.column, 2);
+			const loc2 = smc.originalPositionFor({ line: 1, column: 4 });
+			assert.equal(loc2.line, 1);
+			assert.equal(loc2.column, 2);
+			const loc3 = smc.originalPositionFor({ line: 2, column: 0 });
+			assert.equal(loc3.line, 1);
+			assert.equal(loc3.column, 3);
+			const loc4 = smc.originalPositionFor({ line: 2, column: 1 });
+			assert.equal(loc4.line, 1);
+			assert.equal(loc4.column, 4);
+			// a\nnd\n
+			const loc5 = smc.originalPositionFor({ line: 2, column: 2 });
+			assert.equal(loc5.line, 1);
+			assert.equal(loc5.column, 4);
+			const loc6 = smc.originalPositionFor({ line: 2, column: 3 });
+			assert.equal(loc6.line, 1);
+			assert.equal(loc6.column, 4);
+			const loc7 = smc.originalPositionFor({ line: 3, column: 0 });
+			assert.equal(loc7.line, 1);
+			assert.equal(loc7.column, 4);
+			const loc8 = smc.originalPositionFor({ line: 4, column: 0 });
+			assert.equal(loc8.line, 1);
+			assert.equal(loc8.column, 5);
+		});
 	});
 
 	describe('getIndentString', () => {