lib/goog/string/const.js

1// Copyright 2013 The Closure Library Authors. All Rights Reserved.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// http://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS-IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14
15goog.provide('goog.string.Const');
16
17goog.require('goog.asserts');
18goog.require('goog.string.TypedString');
19
20
21
22/**
23 * Wrapper for compile-time-constant strings.
24 *
25 * Const is a wrapper for strings that can only be created from program
26 * constants (i.e., string literals). This property relies on a custom Closure
27 * compiler check that {@code goog.string.Const.from} is only invoked on
28 * compile-time-constant expressions.
29 *
30 * Const is useful in APIs whose correct and secure use requires that certain
31 * arguments are not attacker controlled: Compile-time constants are inherently
32 * under the control of the application and not under control of external
33 * attackers, and hence are safe to use in such contexts.
34 *
35 * Instances of this type must be created via its factory method
36 * {@code goog.string.Const.from} and not by invoking its constructor. The
37 * constructor intentionally takes no parameters and the type is immutable;
38 * hence only a default instance corresponding to the empty string can be
39 * obtained via constructor invocation.
40 *
41 * @see goog.string.Const#from
42 * @constructor
43 * @final
44 * @struct
45 * @implements {goog.string.TypedString}
46 */
47goog.string.Const = function() {
48 /**
49 * The wrapped value of this Const object. The field has a purposely ugly
50 * name to make (non-compiled) code that attempts to directly access this
51 * field stand out.
52 * @private {string}
53 */
54 this.stringConstValueWithSecurityContract__googStringSecurityPrivate_ = '';
55
56 /**
57 * A type marker used to implement additional run-time type checking.
58 * @see goog.string.Const#unwrap
59 * @const
60 * @private
61 */
62 this.STRING_CONST_TYPE_MARKER__GOOG_STRING_SECURITY_PRIVATE_ =
63 goog.string.Const.TYPE_MARKER_;
64};
65
66
67/**
68 * @override
69 * @const
70 */
71goog.string.Const.prototype.implementsGoogStringTypedString = true;
72
73
74/**
75 * Returns this Const's value a string.
76 *
77 * IMPORTANT: In code where it is security-relevant that an object's type is
78 * indeed {@code goog.string.Const}, use {@code goog.string.Const.unwrap}
79 * instead of this method.
80 *
81 * @see goog.string.Const#unwrap
82 * @override
83 */
84goog.string.Const.prototype.getTypedStringValue = function() {
85 return this.stringConstValueWithSecurityContract__googStringSecurityPrivate_;
86};
87
88
89/**
90 * Returns a debug-string representation of this value.
91 *
92 * To obtain the actual string value wrapped inside an object of this type,
93 * use {@code goog.string.Const.unwrap}.
94 *
95 * @see goog.string.Const#unwrap
96 * @override
97 */
98goog.string.Const.prototype.toString = function() {
99 return 'Const{' +
100 this.stringConstValueWithSecurityContract__googStringSecurityPrivate_ +
101 '}';
102};
103
104
105/**
106 * Performs a runtime check that the provided object is indeed an instance
107 * of {@code goog.string.Const}, and returns its value.
108 * @param {!goog.string.Const} stringConst The object to extract from.
109 * @return {string} The Const object's contained string, unless the run-time
110 * type check fails. In that case, {@code unwrap} returns an innocuous
111 * string, or, if assertions are enabled, throws
112 * {@code goog.asserts.AssertionError}.
113 */
114goog.string.Const.unwrap = function(stringConst) {
115 // Perform additional run-time type-checking to ensure that stringConst is
116 // indeed an instance of the expected type. This provides some additional
117 // protection against security bugs due to application code that disables type
118 // checks.
119 if (stringConst instanceof goog.string.Const &&
120 stringConst.constructor === goog.string.Const &&
121 stringConst.STRING_CONST_TYPE_MARKER__GOOG_STRING_SECURITY_PRIVATE_ ===
122 goog.string.Const.TYPE_MARKER_) {
123 return stringConst.
124 stringConstValueWithSecurityContract__googStringSecurityPrivate_;
125 } else {
126 goog.asserts.fail('expected object of type Const, got \'' +
127 stringConst + '\'');
128 return 'type_error:Const';
129 }
130};
131
132
133/**
134 * Creates a Const object from a compile-time constant string.
135 *
136 * It is illegal to invoke this function on an expression whose
137 * compile-time-contant value cannot be determined by the Closure compiler.
138 *
139 * Correct invocations include,
140 * <pre>
141 * var s = goog.string.Const.from('hello');
142 * var t = goog.string.Const.from('hello' + 'world');
143 * </pre>
144 *
145 * In contrast, the following are illegal:
146 * <pre>
147 * var s = goog.string.Const.from(getHello());
148 * var t = goog.string.Const.from('hello' + world);
149 * </pre>
150 *
151 * TODO(xtof): Compile-time checks that this function is only called
152 * with compile-time constant expressions.
153 *
154 * @param {string} s A constant string from which to create a Const.
155 * @return {!goog.string.Const} A Const object initialized to stringConst.
156 */
157goog.string.Const.from = function(s) {
158 return goog.string.Const.create__googStringSecurityPrivate_(s);
159};
160
161
162/**
163 * Type marker for the Const type, used to implement additional run-time
164 * type checking.
165 * @const {!Object}
166 * @private
167 */
168goog.string.Const.TYPE_MARKER_ = {};
169
170
171/**
172 * Utility method to create Const instances.
173 * @param {string} s The string to initialize the Const object with.
174 * @return {!goog.string.Const} The initialized Const object.
175 * @private
176 */
177goog.string.Const.create__googStringSecurityPrivate_ = function(s) {
178 var stringConst = new goog.string.Const();
179 stringConst.stringConstValueWithSecurityContract__googStringSecurityPrivate_ =
180 s;
181 return stringConst;
182};