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// Copyright 2013 The Closure Library Authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS-IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
goog.provide('goog.string.Const');
goog.require('goog.asserts');
goog.require('goog.string.TypedString');
/**
* Wrapper for compile-time-constant strings.
*
* Const is a wrapper for strings that can only be created from program
* constants (i.e., string literals). This property relies on a custom Closure
* compiler check that {@code goog.string.Const.from} is only invoked on
* compile-time-constant expressions.
*
* Const is useful in APIs whose correct and secure use requires that certain
* arguments are not attacker controlled: Compile-time constants are inherently
* under the control of the application and not under control of external
* attackers, and hence are safe to use in such contexts.
*
* Instances of this type must be created via its factory method
* {@code goog.string.Const.from} and not by invoking its constructor. The
* constructor intentionally takes no parameters and the type is immutable;
* hence only a default instance corresponding to the empty string can be
* obtained via constructor invocation.
*
* @see goog.string.Const#from
* @constructor
* @final
* @struct
* @implements {goog.string.TypedString}
*/
goog.string.Const = function() {
/**
* The wrapped value of this Const object. The field has a purposely ugly
* name to make (non-compiled) code that attempts to directly access this
* field stand out.
* @private {string}
*/
this.stringConstValueWithSecurityContract__googStringSecurityPrivate_ = '';
/**
* A type marker used to implement additional run-time type checking.
* @see goog.string.Const#unwrap
* @const {!Object}
* @private
*/
this.STRING_CONST_TYPE_MARKER__GOOG_STRING_SECURITY_PRIVATE_ =
goog.string.Const.TYPE_MARKER_;
};
/**
* @override
* @const
*/
goog.string.Const.prototype.implementsGoogStringTypedString = true;
/**
* Returns this Const's value a string.
*
* IMPORTANT: In code where it is security-relevant that an object's type is
* indeed {@code goog.string.Const}, use {@code goog.string.Const.unwrap}
* instead of this method.
*
* @see goog.string.Const#unwrap
* @override
*/
goog.string.Const.prototype.getTypedStringValue = function() {
return this.stringConstValueWithSecurityContract__googStringSecurityPrivate_;
};
/**
* Returns a debug-string representation of this value.
*
* To obtain the actual string value wrapped inside an object of this type,
* use {@code goog.string.Const.unwrap}.
*
* @see goog.string.Const#unwrap
* @override
*/
goog.string.Const.prototype.toString = function() {
return 'Const{' +
this.stringConstValueWithSecurityContract__googStringSecurityPrivate_ +
'}';
};
/**
* Performs a runtime check that the provided object is indeed an instance
* of {@code goog.string.Const}, and returns its value.
* @param {!goog.string.Const} stringConst The object to extract from.
* @return {string} The Const object's contained string, unless the run-time
* type check fails. In that case, {@code unwrap} returns an innocuous
* string, or, if assertions are enabled, throws
* {@code goog.asserts.AssertionError}.
*/
goog.string.Const.unwrap = function(stringConst) {
// Perform additional run-time type-checking to ensure that stringConst is
// indeed an instance of the expected type. This provides some additional
// protection against security bugs due to application code that disables type
// checks.
if (stringConst instanceof goog.string.Const &&
stringConst.constructor === goog.string.Const &&
stringConst.STRING_CONST_TYPE_MARKER__GOOG_STRING_SECURITY_PRIVATE_ ===
goog.string.Const.TYPE_MARKER_) {
return stringConst
.stringConstValueWithSecurityContract__googStringSecurityPrivate_;
} else {
goog.asserts.fail(
'expected object of type Const, got \'' + stringConst + '\'');
return 'type_error:Const';
}
};
/**
* Creates a Const object from a compile-time constant string.
*
* It is illegal to invoke this function on an expression whose
* compile-time-contant value cannot be determined by the Closure compiler.
*
* Correct invocations include,
* <pre>
* var s = goog.string.Const.from('hello');
* var t = goog.string.Const.from('hello' + 'world');
* </pre>
*
* In contrast, the following are illegal:
* <pre>
* var s = goog.string.Const.from(getHello());
* var t = goog.string.Const.from('hello' + world);
* </pre>
*
* @param {string} s A constant string from which to create a Const.
* @return {!goog.string.Const} A Const object initialized to stringConst.
*/
goog.string.Const.from = function(s) {
return goog.string.Const.create__googStringSecurityPrivate_(s);
};
/**
* Type marker for the Const type, used to implement additional run-time
* type checking.
* @const {!Object}
* @private
*/
goog.string.Const.TYPE_MARKER_ = {};
/**
* Utility method to create Const instances.
* @param {string} s The string to initialize the Const object with.
* @return {!goog.string.Const} The initialized Const object.
* @private
*/
goog.string.Const.create__googStringSecurityPrivate_ = function(s) {
var stringConst = new goog.string.Const();
stringConst.stringConstValueWithSecurityContract__googStringSecurityPrivate_ =
s;
return stringConst;
};
/**
* A Const instance wrapping the empty string.
* @const {!goog.string.Const}
*/
goog.string.Const.EMPTY = goog.string.Const.from('');
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