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Closure Compiler is a JavaScript optimizing compiler. It parses your
JavaScript, analyzes it, removes dead code and rewrites and minimizes
what's left. It also checks syntax, variable references, and types, and
warns about common JavaScript pitfalls. It is used in many of Google's
JavaScript apps, including Gmail, Google Web Search, Google Maps, and
Google Docs.
/*
* Copyright 2016 The Closure Compiler Authors.
*
* 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.
*/
/**
* @fileoverview Provides methods to polyfill native objects.
* @suppress {uselessCode}
*/
'require util/defineproperty';
'require util/defines';
'require util/global';
'require util/shouldpolyfill';
/** @const {!Object} map from classes (Map) to polyfills*/
$jscomp.polyfills = {};
/** @const {!Object} */
$jscomp.propertyToPolyfillSymbol = {};
/** @const {string} */
$jscomp.POLYFILL_PREFIX = '$jscp$';
/**
* Returns a polyfilled version of the given property if present, otherwise
* returns the actual property.
*
* This is a variable instead of a $jscomp.* property to make it simpler for
* the compiler to avoid prematurely deleting it during optimizations.
*
* @param {*} target the receiver for the property access, e.g. `my.str`
* @param {string} key the name of the property, e.g. `includes`
* @return {?} if a polyfill is present, that polyfill property. otherwise
* the result of target[key]
* @noinline prevent inlining so IsolatePolyfills can find this declaration.
* @suppress {reportUnknownTypes}
*/
var $jscomp$lookupPolyfilledValue = function(target, key) {
/** @const */
var polyfilledKey = $jscomp.propertyToPolyfillSymbol[key];
if (polyfilledKey == null) {
return target[key];
}
// NOTE: this will throw if `target` is null or undefined.
/** @const */
var polyfill = target[polyfilledKey];
// Checking `polyfill !== undefined` is sufficient because $jscomp.polyfill
// never installs nullish polyfills. Otherwise we'd have to check something
// like `key in target`.
return polyfill !== undefined ? polyfill : target[key];
};
/**
* @param {string} target Qualified name of the class or method to polyfill,
* e.g. 'Array.prototype.includes' or 'Map'.
* @param {?function(*): *} polyfill A function that takes the current browser
* implementation of the target and returns an optional new polyfill
* implementation. If null is returned, then no polyfill will be added. A
* null argument for this parameter indicates that the function will not be
* polyfilled, and is only useful for `build_polyfill_table.js` bookkeeping.
* @param {string} fromLang The language level in which the target is expected
* to already be present in the browser. The compiler requires that
* `languageOut < fromLang` before injecting a polyfill (i.e. if the
* specified output language already includes the feature then there's no
* need to polyfill it).
* @param {string} toLang The language level required by the polyfill
* implementation. The compiler will issue an error if a polyfill is
* required, but `languageOut < toLang`. Additionally, the
* `build_polyfill_table.js` script audits the polyfill dependency tree to
* ensure that no polyfill with a lower `toLang` depends on one with a
* higher `toLang`.
* @noinline
* NOTE: We prevent inlining so RemoveUnusedPolyfills can always recognize this
* call.
*/
$jscomp.polyfill = function(target, polyfill, fromLang, toLang) {
if (!polyfill) return;
if ($jscomp.ISOLATE_POLYFILLS) {
$jscomp.polyfillIsolated(target, polyfill, fromLang, toLang);
} else {
$jscomp.polyfillUnisolated(target, polyfill, fromLang, toLang);
}
};
/**
* @param {string} target Qualified name of the class or method to polyfill,
* @param {function(*): *} polyfill A function that takes the current browser
* implementation of the target and returns an optional new polyfill
* implementation.
* @param {string} fromLang The language level in which the target is expected
* to already be present in the browser.
* @param {string} toLang The language level required by the polyfill
* implementation.
* @suppress {reportUnknownTypes}
* @see $jscomp.polyfill for more documentation
*/
$jscomp.polyfillUnisolated = function(target, polyfill, fromLang, toLang) {
var obj = $jscomp.global;
var split = target.split('.');
for (var i = 0; i < split.length - 1; i++) {
var key = split[i];
if (!(key in obj)) obj[key] = {}; // Might want to be defineProperty.
obj = obj[key];
}
var property = split[split.length - 1];
var orig = obj[property];
var impl = polyfill(orig);
if (impl == orig || impl == null) return;
$jscomp.defineProperty(
obj, property, {configurable: true, writable: true, value: impl});
};
/**
* Defines a polyfill for the given class or method under an obfuscated name.
*
* The main differences between this method and $jscomp.polyfillUnisolated are:
* - classes are defined on $jscomp.polyfills, not window.
* - methods (Array.prototype.includes) are defined on Array.prototype under
* an obfuscated name Array.prototype.$jscp$includes or a Symbol, if native.
* - this method installs our polyfill even when an existing implementation is
* found, as it might be an untrusted polyfill. The exception is that if
* Symbol is detected to be native, we assume any implementations of ES6
* classes/methods are also native and not polyfills.
*
* @param {string} target Qualified name of the class or method to polyfill,
* @param {function(*): *} polyfill A function that takes the current browser
* implementation of the target and returns an optional new polyfill
* implementation.
* @param {string} fromLang The language level in which the target is expected
* to already be present in the browser.
* @param {string} toLang The language level required by the polyfill
* implementation.
* @suppress {reportUnknownTypes}
* @see $jscomp.polyfill for more parameter documentation
*/
$jscomp.polyfillIsolated = function(target, polyfill, fromLang, toLang) {
var split = target.split('.');
var isNativeClass = split.length === 1;
var root = split[0];
// Look up the parent object. For classes this is just $jscomp.global.
// For methods/properties this may be a polyfill (Promise) or may a native
// object (Array.prototype).
var obj;
if (!isNativeClass && root in $jscomp.polyfills) {
// Handle Symbol.asyncIterator and Promise.prototype.finally
obj = $jscomp.polyfills;
} else {
// Handle Promise, Array.of, and String.prototype.startsWith
obj = $jscomp.global;
}
for (var i = 0; i < split.length - 1; i++) {
var key = split[i];
if (!(key in obj)) obj[key] = {}; // Might want to be defineProperty.
obj = obj[key];
}
var property = split[split.length - 1];
// If Symbol is native and the target is in the ES6 spec, use the native imp.
// We assume the method/class was not polyfilled since polyfills typically
// back off in the presence of an existing implementation.
var nativeImpl =
$jscomp.IS_SYMBOL_NATIVE && fromLang === 'es6' ? obj[property] : null;
var impl = polyfill(nativeImpl);
if (impl == null) {
return;
}
if (isNativeClass) {
// Note: `impl` may be the actual native class instead of a
// polyfill. Add it to $jscomp.polyfills anyway. The IsolatePolyfills pass
// unconditionally replaces `Symbol` with `$jscomp.polyfills['Symbol']`.
$jscomp.defineProperty(
$jscomp.polyfills, property,
{configurable: true, writable: true, value: impl});
} else if (impl !== nativeImpl) {
// Skip installing an obfuscated property if we have found a native version
// of the method we're polyfilling. $jscomp$lookupPolyfilledValue will fall
// back to the native version anyway.
$jscomp.propertyToPolyfillSymbol[property] = $jscomp.IS_SYMBOL_NATIVE ?
// use bracket access to avoid injecting the Symbol polyfill
$jscomp.global['Symbol'](property) :
$jscomp.POLYFILL_PREFIX + property;
property = $jscomp.propertyToPolyfillSymbol[property];
$jscomp.defineProperty(
obj, property, {configurable: true, writable: true, value: impl});
}
};