Allow intercept of transitive require calls files.

This paves the way for avajs/ava#156

index.js

@@ -21,6 +21,11 @@ module.exports = function requireFromString(code, filename, opts) {
 	var paths = Module._nodeModulePaths(path.dirname(filename));
 
 	var m = new Module(filename, module.parent);
+	if (opts.require) {
+		m.require = function (path) {
+			return opts.require.call(this, path);
+		}
+	}
 	m.filename = filename;
 	m.paths = [].concat(opts.prependPaths).concat(paths).concat(opts.appendPaths);
 	m._compile(code, filename);

test/fixture/depth0.js

@@ -0,0 +1,2 @@
+
+module.exports = require('./depth1');

test/fixture/depth1/depth2/index.js

@@ -0,0 +1 @@
+module.exports = 'FOO - depth2';

test/fixture/depth1/index.js

@@ -0,0 +1 @@
+module.exports = require('./depth2');

test/fixture/greet-james.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+	return 'Hello ' + require('./james');
+};

test/fixture/james.js

@@ -0,0 +1 @@
+module.exports = 'James';

test/index.js

@@ -7,6 +7,13 @@ var fs = require('fs');
 var path = require('path');
 var requireFromString = require('../');
 
+function getFixture(file) {
+	file = path.join(__dirname, 'fixture', file);
+	var code = fs.readFileSync(file, 'utf8');
+
+	return {file: file, code: code};
+}
+
 it('should accept only string as code', function () {
 	assert.throws(function () {
 		requireFromString();
@@ -24,18 +31,16 @@ it('should accept filename', function () {
 });
 
 it('should work with relative require in file', function () {
-	var file = path.join(__dirname, '/fixture/module.js');
-	var code = fs.readFileSync(file, 'utf8');
-	var result = requireFromString(code, file);
+	var fixture = getFixture('module.js');
+	var result = requireFromString(fixture.code, fixture.file);
 
 	assert.ok(result);
 	assert.ok(module === result.parent.parent);
 });
 
 it('should have appended and preppended paths', function () {
-	var file = path.join(__dirname, '/fixture/submodule.js');
-	var code = fs.readFileSync(file, 'utf8');
-	var result = requireFromString(code, file, {
+	var fixture = getFixture('submodule.js');
+	var result = requireFromString(fixture.code, fixture.file, {
 		appendPaths: ['append'],
 		prependPaths: ['prepend']
 	});
@@ -44,3 +49,41 @@ it('should have appended and preppended paths', function () {
 	assert.equal(result.paths.indexOf('append'), result.paths.length - 1);
 	assert.equal(result.paths.indexOf('prepend'), 0);
 });
+
+it('should allow modification of other required modules via callback', function () {
+	var fixture = getFixture('greet-james.js');
+	var Module = module.constructor;
+
+	function transform(code) {
+		return code.replace('James', 'Jim');
+	}
+
+	function requireHook(path) {
+		var file = Module._resolveFilename(path, this);
+		var code = fs.readFileSync(file, 'utf8');
+		return requireFromString(transform(code), file, {require: requireHook});
+	}
+
+	var result = requireFromString(fixture.code, fixture.file, {require: requireHook});
+
+	assert.equal(result(), 'Hello Jim');
+});
+
+it('transforms should be doable even as relative directory changes', function () {
+	var fixture = getFixture('depth0.js');
+	var Module = module.constructor;
+
+	function transform(code) {
+		return code.replace('FOO', 'BAR');
+	}
+
+	function requireHook(path) {
+		var file = Module._resolveFilename(path, this);
+		var code = fs.readFileSync(file, 'utf8');
+		return requireFromString(transform(code), file, {require: requireHook});
+	}
+
+	var result = requireFromString(fixture.code, fixture.file, {require: requireHook});
+
+	assert.equal(result, 'BAR - depth2');
+});