wp-calypso/client/lib/local-storage-bypass/index.js

import debugModule from 'debug';

const debug = debugModule( 'calypso:support-user' );

export const length = ( memoryStore ) => {
	return () => {
		debug( 'Bypassing localStorage', 'length property' );
		return Object.keys( memoryStore ).length;
	};
};

export const key = ( memoryStore ) => {
	return ( index ) => {
		debug( 'Bypassing localStorage', 'key' );
		if ( index >= Object.keys( memoryStore ).length ) {
			return null;
		}

		return Object.keys( memoryStore )[ index ];
	};
};

export const setItem = ( memoryStore, allowedKeys, original ) => {
	return ( _key, value ) => {
		if ( original && allowedKeys.includes( _key ) ) {
			original( _key, value );
			return;
		}

		debug( 'Bypassing localStorage', 'setItem', _key );
		memoryStore[ _key ] = value;
	};
};

export const getItem = ( memoryStore, allowedKeys, original ) => {
	return ( _key ) => {
		if ( original && allowedKeys.includes( _key ) ) {
			return original( _key );
		}

		debug( 'Bypassing localStorage', 'getItem', _key );
		return memoryStore[ _key ] || null;
	};
};

export const removeItem = ( memoryStore, allowedKeys, original ) => {
	return ( _key ) => {
		if ( original && allowedKeys.includes( _key ) ) {
			original( _key );
			return;
		}

		debug( 'Bypassing localStorage', 'removeItem', _key );
		delete memoryStore[ _key ];
	};
};

export const clear = ( memoryStore ) => {
	return () => {
		debug( 'Bypassing localStorage', 'clear' );

		for ( const _key in memoryStore ) {
			delete memoryStore[ _key ];
		}
	};
};

/**
 * Overrides localStorage, using an in-memory store instead of the real localStorage.
 *
 * This function replaces localStorage with an in-memory store and is useful in the following cases:
 * 1. Avoiding conflicts caused by shared localStorage across multiple support user sessions.
 * 2. Providing a working localStorage implementation for older Safari versions that throw errors in Private mode.
 * @param {Object}   [args]            An arguments object
 * @param {Object}   [args.root]       Allow alternate "window" object to support tests in non-browser environments
 * @param {string[]} [args.allowedKeys] An array of localStorage keys that are proxied to the real localStorage
 */
export default function localStorageBypass( {
	root = typeof window === 'undefined' ? undefined : window,
	allowedKeys = [],
} = {} ) {
	debug( 'Bypassing localStorage' );

	let _setItem;
	let _getItem;
	let _removeItem;

	if ( root && root.localStorage && root.Storage && root.Storage.prototype ) {
		_setItem = root.Storage.prototype.setItem.bind( root.localStorage );
		_getItem = root.Storage.prototype.getItem.bind( root.localStorage );
		_removeItem = root.Storage.prototype.removeItem.bind( root.localStorage );
	}

	const memoryStore = {};
	const getMemoryStoreLength = length( memoryStore );
	const localStorageOverride = {
		setItem: setItem( memoryStore, allowedKeys, _setItem ),
		getItem: getItem( memoryStore, allowedKeys, _getItem ),
		removeItem: removeItem( memoryStore, allowedKeys, _removeItem ),
		key: key( memoryStore ),
		clear: clear( memoryStore ),
	};

	try {
		// Try redefining `window.localStorage` instead of assigning to localStorage methods
		// like `getItem` and `setItem` because it is not effective in Firefox.
		// https://github.com/whatwg/html/issues/183#issuecomment-142944605
		//
		// NOTE: Based on testing, Safari versions 5-9 throw an error when attempting to redefine localStorage.
		Object.defineProperty( root, 'localStorage', {
			value: localStorageOverride,
			enumerable: true,
			configurable: true,
		} );
		Object.defineProperty( root.localStorage, 'length', { get: getMemoryStoreLength } );
	} catch ( error ) {
		// Attempt to assign to localStorage methods as a last effort.
		// This is necessary in desktop Safari versions 6-9 as they do not allow
		// window.localStorage to be redefined.
		Object.assign( root.localStorage, localStorageOverride );

		// ATTENTION: No `length` getter is provided in this case, so
		// localStorage.length will always be zero. Previous implementations used
		// localStorage.__defineGetter__ which does not throw but also is ineffective.
		// This behavior was observed in testing Safari versions 6-12.
	}
}