Fix map resolution

You prefix `source` with `sourceRoot`, then resolve it relative to the map URL. Before we incorrectly allowed a map's source to be absolute after the `sourceRoot` was specified.

This matches `source-map` and Chrome's behaviors.

src/resolve.ts

@@ -1,10 +1,16 @@
 import resolveUri from '@jridgewell/resolve-uri';
+import stripFilename from './strip-filename';
 
-export default function resolve(input: string, base: string | undefined): string {
-  // The base is always treated as a directory, if it's not empty.
+type Resolve = (source: string | null) => string;
+export default function resolver(
+  mapUrl: string | null | undefined,
+  sourceRoot: string | undefined,
+): Resolve {
+  const from = stripFilename(mapUrl);
+  // The sourceRoot is always treated as a directory, if it's not empty.
   // https://github.com/mozilla/source-map/blob/8cb3ee57/lib/util.js#L327
   // https://github.com/chromium/chromium/blob/da4adbb3/third_party/blink/renderer/devtools/front_end/sdk/SourceMap.js#L400-L401
-  if (base && !base.endsWith('/')) base += '/';
+  const prefix = sourceRoot ? sourceRoot + '/' : '';
 
-  return resolveUri(input, base);
+  return (source) => resolveUri(prefix + (source || ''), from);
 }

src/trace-mapping.ts

@@ -1,7 +1,6 @@
 import { encode, decode } from '@jridgewell/sourcemap-codec';
 
-import resolve from './resolve';
-import stripFilename from './strip-filename';
+import resolver from './resolve';
 import maybeSort from './sort';
 import buildBySources from './by-source';
 import {
@@ -119,8 +118,8 @@ export class TraceMap implements SourceMap {
     this.sourcesContent = sourcesContent;
     this.ignoreList = parsed.ignoreList || (parsed as XInput).x_google_ignoreList || undefined;
 
-    const from = resolve(sourceRoot || '', stripFilename(mapUrl));
-    this.resolvedSources = sources.map((s) => resolve(s || '', from));
+    const resolve = resolver(mapUrl, sourceRoot);
+    this.resolvedSources = sources.map((s) => resolve(s));
 
     const { mappings } = parsed;
     if (typeof mappings === 'string') {

src/types.ts

@@ -20,10 +20,7 @@ export interface DecodedSourceMap extends SourceMapV3 {
 }
 
 export interface Section {
-  offset: {
-    line: number;
-    column: number;
-  };
+  offset: { line: number; column: number };
   map: EncodedSourceMap | DecodedSourceMap | SectionedSourceMap;
 }
 

test/resolve.test.ts

@@ -1,18 +1,34 @@
 import { strict as assert } from 'assert';
-import resolve from '../src/resolve';
+import resolver from '../src/resolve';
 
 describe('resolve', () => {
-  it('resolves input relative to base', () => {
-    const base = 'bar/';
-    const input = 'foo';
+  it('unresolved without sourceRoot', () => {
+    const resolve = resolver(undefined, undefined);
+    assert.equal(resolve('input.js'), 'input.js');
+  });
+
+  it('relative to mapUrl', () => {
+    const resolve = resolver('foo/script.js.map', undefined);
+    assert.equal(resolve('input.js'), 'foo/input.js');
+  });
 
-    assert.equal(resolve(input, base), 'bar/foo');
+  it('relative to sourceRoot', () => {
+    const resolve = resolver(undefined, 'foo');
+    assert.equal(resolve('input.js'), 'foo/input.js');
   });
 
-  it('treats base as a directory regardless of slash', () => {
-    const base = 'bar';
-    const input = 'foo';
+  it('relative to mapUrl then sourceRoot', () => {
+    const resolve = resolver('foo/script.js.map', 'bar');
+    assert.equal(resolve('input.js'), 'foo/bar/input.js');
+  });
+
+  it('prepends sourceRoot to source before resolving', () => {
+    const resolve = resolver('foo/script.js.map', 'bar');
+    assert.equal(resolve('/input.js'), 'foo/bar/input.js');
+  });
 
-    assert.equal(resolve(input, base), 'bar/foo');
+  it('skips undefined sourceRoot before resolving', () => {
+    const resolve = resolver('foo/script.js.map', undefined);
+    assert.equal(resolve('/input.js'), '/input.js');
   });
 });

test/trace-mapping.test.ts

@@ -31,8 +31,7 @@ describe('TraceMap', () => {
   const decodedMap: DecodedSourceMap = {
     version: 3,
     sources: ['input.js'],
-    sourceRoot:
-      'https://astexplorer.net/#/gist/d91f04e37e8e12eec06f2886e6bc3a4d/56cd06cd895d3b638b4100658b0027787ca5e5f1',
+    sourceRoot: 'https://astexplorer.net/',
     names: ['foo', 'bar', 'Error'],
     mappings: [
       [