diff --git a/src/filters-downloader-creator.js b/src/filters-downloader-creator.js
index cdc1fc0..45237d9 100644
--- a/src/filters-downloader-creator.js
+++ b/src/filters-downloader-creator.js
@@ -31,7 +31,8 @@
 const FiltersDownloaderCreator = (FileDownloadWrapper) => {
     "use strict";
 
-    const CONDITION_DIRECTIVE_START = "!#if";
+    const CONDITION_IF_DIRECTIVE_START = "!#if";
+    const CONDITION_ELSE_DIRECTIVE_START = "!#else";
     const CONDITION_DIRECTIVE_END = "!#endif";
 
     const CONDITION_OPERATOR_NOT = "!";
@@ -75,19 +76,20 @@ const FiltersDownloaderCreator = (FileDownloadWrapper) => {
     };
 
     /**
-     * Finds end of condition block started with startIndex
+     * Finds end of condition block started with startIndex.
      *
-     * @param rules
-     * @param startIndex
+     * @param {string[]} rules Array of all rules.
+     * @param {number} startIndex Index of the start of the condition block.
+     *
+     * @returns {number} Index of the end of the condition block.
      */
     const findConditionEnd = (rules, startIndex) => {
         const stack = [];
         for (let j = startIndex; j < rules.length; j++) {
             let internalRule = rules[j];
 
-            if (internalRule.startsWith(CONDITION_DIRECTIVE_START)) {
-                stack.push(CONDITION_DIRECTIVE_START);
-
+            if (internalRule.startsWith(CONDITION_IF_DIRECTIVE_START)) {
+                stack.push(CONDITION_IF_DIRECTIVE_START);
             } else if (internalRule.startsWith(CONDITION_DIRECTIVE_END)) {
                 if (stack.length > 0) {
                     stack.pop();
@@ -101,10 +103,14 @@ const FiltersDownloaderCreator = (FileDownloadWrapper) => {
     };
 
     /**
-     * Resolves constant expression
+     * Resolves constant expression.
      *
-     * @param expression
-     * @param definedProperties
+     * @param {string} expression
+     * @param {object} [definedProperties]
+     *
+     * @returns {boolean} True if expression is `'true'`
+     * or there is a property with the same name in `definedProperties` which is set to true,
+     * false otherwise.
      */
     const resolveConditionConstant = (expression, definedProperties) => {
         if (!expression) {
@@ -118,8 +124,11 @@ const FiltersDownloaderCreator = (FileDownloadWrapper) => {
     /**
      * Calculates conditional expression
      *
-     * @param expression
-     * @param definedProperties
+     * @param {string} expression Conditional expression.
+     * @param {object} [definedProperties] Object with the defined properties
+     * where keys are platform names and values are booleans.
+     *
+     * @returns {boolean} Result of the expression
      */
     const resolveExpression = (expression, definedProperties) => {
         if (!expression) {
@@ -152,11 +161,11 @@ const FiltersDownloaderCreator = (FileDownloadWrapper) => {
         const indexOfNotOperator = expression.indexOf(CONDITION_OPERATOR_NOT);
 
         if (indexOfOrOperator !== -1) {
-            result = resolveExpression(expression.substring(0, indexOfOrOperator - 1), definedProperties) ||
-                resolveExpression(expression.substring(indexOfOrOperator + CONDITION_OPERATOR_OR.length, expression.length), definedProperties);
+            result = resolveExpression(expression.substring(0, indexOfOrOperator - 1), definedProperties)
+                || resolveExpression(expression.substring(indexOfOrOperator + CONDITION_OPERATOR_OR.length, expression.length), definedProperties);
         } else if (indexOfAndOperator !== -1) {
-            result = resolveExpression(expression.substring(0, indexOfAndOperator - 1), definedProperties) &&
-                resolveExpression(expression.substring(indexOfAndOperator + CONDITION_OPERATOR_AND.length, expression.length), definedProperties);
+            result = resolveExpression(expression.substring(0, indexOfAndOperator - 1), definedProperties)
+                && resolveExpression(expression.substring(indexOfAndOperator + CONDITION_OPERATOR_AND.length, expression.length), definedProperties);
         } else if (indexOfNotOperator === 0) {
             result = !resolveExpression(expression.substring(CONDITION_OPERATOR_NOT.length), definedProperties);
         } else {
@@ -173,7 +182,7 @@ const FiltersDownloaderCreator = (FileDownloadWrapper) => {
      * @param definedProperties
      */
     const resolveCondition = (directive, definedProperties) => {
-        const expression = directive.substring(CONDITION_DIRECTIVE_START.length).trim();
+        const expression = directive.substring(CONDITION_IF_DIRECTIVE_START.length).trim();
 
         return resolveExpression(expression, definedProperties);
     };
@@ -181,8 +190,10 @@ const FiltersDownloaderCreator = (FileDownloadWrapper) => {
     /**
      * Resolves conditions directives
      *
-     * @param rules
-     * @param definedProperties
+     * @param {string} rules Input array of rules.
+     * @param {object} [definedProperties] Object with the defined properties.
+     *
+     * @returns {string[]} Array of rules.
      */
     const resolveConditions = (rules, definedProperties) => {
         if (!definedProperties) {
@@ -192,17 +203,17 @@ const FiltersDownloaderCreator = (FileDownloadWrapper) => {
         let result = [];
 
         for (let i = 0; i < rules.length; i++) {
-            let rule = rules[i];
+            const rule = rules[i];
 
-            if (rule.indexOf(CONDITION_DIRECTIVE_START) === 0) {
+            if (rule.indexOf(CONDITION_IF_DIRECTIVE_START) === 0) {
                 let endLineIndex = findConditionEnd(rules, i + 1);
                 if (endLineIndex === -1) {
                     throw new Error('Invalid directives: Condition end not found: ' + rule);
                 }
 
-                let conditionValue = resolveCondition(rule, definedProperties);
-                if (conditionValue) {
-                    let rulesUnderCondition = rules.slice(i + 1, endLineIndex);
+                const isConditionMatched = resolveCondition(rule, definedProperties);
+                if (isConditionMatched) {
+                    const rulesUnderCondition = rules.slice(i + 1, endLineIndex);
                     // Resolve inner conditions in recursion
                     result = result.concat(resolveConditions(rulesUnderCondition, definedProperties));
                 }
@@ -287,12 +298,15 @@ const FiltersDownloaderCreator = (FileDownloadWrapper) => {
     };
 
     /**
-     * Compiles filter content
+     * Compiles filter content.
      *
-     * @param {Array} rules Array of strings
-     * @param {?string} filterOrigin Filter file URL origin or null
-     * @param {?object} definedProperties An object with the defined properties. These properties might be used in pre-processor directives (`#if`, etc)
-     * @returns {Promise} A promise that returns {string} with rules when if resolved and {Error} if rejected.
+     * @param {Array} rules Array of strings.
+     * @param {string} [filterOrigin] Filter file URL origin or null.
+     * @param {object} [definedProperties] An object with the defined properties.
+     * These properties might be used in pre-processor directives (`#if`, etc).
+     *
+     * @returns {Promise} A promise that returns `string` with rules when if resolved
+     * and `Error` if rejected.
      */
     const compile = (rules, filterOrigin, definedProperties) => {
         try {