url: return backslashes from fileURLToPath on Windows

lib/internal/url.js

@@ -1275,6 +1275,8 @@ function urlToOptions(url) {
   return options;
 }
 
+const forwardSlashRegEx = /\//g;
+
 function getPathFromURLWin32(url) {
   var hostname = url.hostname;
   var pathname = url.pathname;
@@ -1289,6 +1291,7 @@ function getPathFromURLWin32(url) {
       }
     }
   }
+  pathname = pathname.replace(forwardSlashRegEx, '\\');
   pathname = decodeURIComponent(pathname);
   if (hostname !== '') {
     // If hostname is set, then we have a UNC path
@@ -1297,7 +1300,7 @@ function getPathFromURLWin32(url) {
     // about percent encoding because the URL parser will have
     // already taken care of that for us. Note that this only
     // causes IDNs with an appropriate `xn--` prefix to be decoded.
-    return `//${domainToUnicode(hostname)}${pathname}`;
+    return `\\\\${domainToUnicode(hostname)}${pathname}`;
   } else {
     // Otherwise, it's a local path that requires a drive letter
     var letter = pathname.codePointAt(1) | 0x20;

test/parallel/test-fs-whatwg-url.js

@@ -63,7 +63,7 @@ if (common.isWindows) {
       code: 'ERR_INVALID_ARG_VALUE',
       type: TypeError,
       message: 'The argument \'path\' must be a string or Uint8Array without ' +
-               'null bytes. Received \'c:/tmp/\\u0000test\''
+               'null bytes. Received \'c:\\\\tmp\\\\\\u0000test\''
     }
   );
 } else {

test/parallel/test-url-fileurltopath.js

@@ -0,0 +1,152 @@
+'use strict';
+const { isWindows } = require('../common');
+const assert = require('assert');
+const url = require('url');
+
+function testInvalidArgs(...args) {
+  for (const arg of args) {
+    assert.throws(() => url.fileURLToPath(arg), {
+      code: 'ERR_INVALID_ARG_TYPE'
+    });
+  }
+}
+
+// Input must be string or URL
+testInvalidArgs(null, undefined, 1, {}, true);
+
+// Input must be a file URL
+assert.throws(() => url.fileURLToPath('https://a/b/c'), {
+  code: 'ERR_INVALID_URL_SCHEME'
+});
+
+{
+  const withHost = new URL('file://host/a');
+
+  if (isWindows) {
+    assert.strictEqual(url.fileURLToPath(withHost), '\\\\host\\a');
+  } else {
+    assert.throws(() => url.fileURLToPath(withHost), {
+      code: 'ERR_INVALID_FILE_URL_HOST'
+    });
+  }
+}
+
+{
+  if (isWindows) {
+    assert.throws(() => url.fileURLToPath('file:///C:/a%2F/'), {
+      code: 'ERR_INVALID_FILE_URL_PATH'
+    });
+    assert.throws(() => url.fileURLToPath('file:///C:/a%5C/'), {
+      code: 'ERR_INVALID_FILE_URL_PATH'
+    });
+    assert.throws(() => url.fileURLToPath('file:///?:/'), {
+      code: 'ERR_INVALID_FILE_URL_PATH'
+    });
+  } else {
+    assert.throws(() => url.fileURLToPath('file:///a%2F/'), {
+      code: 'ERR_INVALID_FILE_URL_PATH'
+    });
+  }
+}
+
+{
+  let testCases;
+  if (isWindows) {
+    testCases = [
+      // lowercase ascii alpha
+      { path: 'C:\\foo', fileURL: 'file:///C:/foo' },
+      // uppercase ascii alpha
+      { path: 'C:\\FOO', fileURL: 'file:///C:/FOO' },
+      // dir
+      { path: 'C:\\dir\\foo', fileURL: 'file:///C:/dir/foo' },
+      // trailing separator
+      { path: 'C:\\dir\\', fileURL: 'file:///C:/dir/' },
+      // dot
+      { path: 'C:\\foo.mjs', fileURL: 'file:///C:/foo.mjs' },
+      // space
+      { path: 'C:\\foo bar', fileURL: 'file:///C:/foo%20bar' },
+      // question mark
+      { path: 'C:\\foo?bar', fileURL: 'file:///C:/foo%3Fbar' },
+      // number sign
+      { path: 'C:\\foo#bar', fileURL: 'file:///C:/foo%23bar' },
+      // ampersand
+      { path: 'C:\\foo&bar', fileURL: 'file:///C:/foo&bar' },
+      // equals
+      { path: 'C:\\foo=bar', fileURL: 'file:///C:/foo=bar' },
+      // colon
+      { path: 'C:\\foo:bar', fileURL: 'file:///C:/foo:bar' },
+      // semicolon
+      { path: 'C:\\foo;bar', fileURL: 'file:///C:/foo;bar' },
+      // percent
+      { path: 'C:\\foo%bar', fileURL: 'file:///C:/foo%25bar' },
+      // backslash
+      { path: 'C:\\foo\\bar', fileURL: 'file:///C:/foo/bar' },
+      // backspace
+      { path: 'C:\\foo\bbar', fileURL: 'file:///C:/foo%08bar' },
+      // tab
+      { path: 'C:\\foo\tbar', fileURL: 'file:///C:/foo%09bar' },
+      // newline
+      { path: 'C:\\foo\nbar', fileURL: 'file:///C:/foo%0Abar' },
+      // carriage return
+      { path: 'C:\\foo\rbar', fileURL: 'file:///C:/foo%0Dbar' },
+      // latin1
+      { path: 'C:\\fóóbàr', fileURL: 'file:///C:/f%C3%B3%C3%B3b%C3%A0r' },
+      // euro sign (BMP code point)
+      { path: 'C:\\€', fileURL: 'file:///C:/%E2%82%AC' },
+      // rocket emoji (non-BMP code point)
+      { path: 'C:\\🚀', fileURL: 'file:///C:/%F0%9F%9A%80' }
+    ];
+  } else {
+    testCases = [
+      // lowercase ascii alpha
+      { path: '/foo', fileURL: 'file:///foo' },
+      // uppercase ascii alpha
+      { path: '/FOO', fileURL: 'file:///FOO' },
+      // dir
+      { path: '/dir/foo', fileURL: 'file:///dir/foo' },
+      // trailing separator
+      { path: '/dir/', fileURL: 'file:///dir/' },
+      // dot
+      { path: '/foo.mjs', fileURL: 'file:///foo.mjs' },
+      // space
+      { path: '/foo bar', fileURL: 'file:///foo%20bar' },
+      // question mark
+      { path: '/foo?bar', fileURL: 'file:///foo%3Fbar' },
+      // number sign
+      { path: '/foo#bar', fileURL: 'file:///foo%23bar' },
+      // ampersand
+      { path: '/foo&bar', fileURL: 'file:///foo&bar' },
+      // equals
+      { path: '/foo=bar', fileURL: 'file:///foo=bar' },
+      // colon
+      { path: '/foo:bar', fileURL: 'file:///foo:bar' },
+      // semicolon
+      { path: '/foo;bar', fileURL: 'file:///foo;bar' },
+      // percent
+      { path: '/foo%bar', fileURL: 'file:///foo%25bar' },
+      // backslash
+      { path: '/foo\\bar', fileURL: 'file:///foo%5Cbar' },
+      // backspace
+      { path: '/foo\bbar', fileURL: 'file:///foo%08bar' },
+      // tab
+      { path: '/foo\tbar', fileURL: 'file:///foo%09bar' },
+      // newline
+      { path: '/foo\nbar', fileURL: 'file:///foo%0Abar' },
+      // carriage return
+      { path: '/foo\rbar', fileURL: 'file:///foo%0Dbar' },
+      // latin1
+      { path: '/fóóbàr', fileURL: 'file:///f%C3%B3%C3%B3b%C3%A0r' },
+      // euro sign (BMP code point)
+      { path: '/€', fileURL: 'file:///%E2%82%AC' },
+      // rocket emoji (non-BMP code point)
+      { path: '/🚀', fileURL: 'file:///%F0%9F%9A%80' },
+    ];
+  }
+
+  for (const { path, fileURL } of testCases) {
+    const fromString = url.fileURLToPath(fileURL);
+    assert.strictEqual(fromString, path);
+    const fromURL = url.fileURLToPath(new URL(fileURL));
+    assert.strictEqual(fromURL, path);
+  }
+}