diff --git a/ext/bg/js/api.js b/ext/bg/js/api.js
index b489b8d2..c43a6dd7 100644
--- a/ext/bg/js/api.js
+++ b/ext/bg/js/api.js
@@ -87,7 +87,7 @@ async function apiTextParse(text, optionsContext) {
     while (text.length > 0) {
         const term = [];
         const [definitions, sourceLength] = await translator.findTermsInternal(
-            text.slice(0, options.scanning.length),
+            text.substring(0, options.scanning.length),
             dictEnabledSet(options),
             options.scanning.alphanumeric,
             {}
@@ -95,16 +95,16 @@ async function apiTextParse(text, optionsContext) {
         if (definitions.length > 0) {
             dictTermsSort(definitions);
             const {expression, reading} = definitions[0];
-            const source = text.slice(0, sourceLength);
+            const source = text.substring(0, sourceLength);
             for (const {text, furigana} of jpDistributeFuriganaInflected(expression, reading, source)) {
                 const reading = jpConvertReading(text, furigana, options.parsing.readingMode);
                 term.push({text, reading});
             }
-            text = text.slice(source.length);
+            text = text.substring(source.length);
         } else {
             const reading = jpConvertReading(text[0], null, options.parsing.readingMode);
             term.push({text: text[0], reading});
-            text = text.slice(1);
+            text = text.substring(1);
         }
         results.push(term);
     }
diff --git a/ext/bg/js/search-query-parser.js b/ext/bg/js/search-query-parser.js
index 1c583bf1..1632b561 100644
--- a/ext/bg/js/search-query-parser.js
+++ b/ext/bg/js/search-query-parser.js
@@ -148,10 +148,9 @@ class QueryParser {
 
     async setPreview(text) {
         const previewTerms = [];
-        while (text.length > 0) {
-            const tempText = text.slice(0, 2);
-            previewTerms.push([{text: Array.from(tempText)}]);
-            text = text.slice(2);
+        for (let i = 0, ii = text.length; i < ii; i += 2) {
+            const tempText = text.substring(i, i + 2);
+            previewTerms.push([{text: tempText.split('')}]);
         }
         this.queryParser.innerHTML = await apiTemplateRender('query-parser.html', {
             terms: previewTerms,
diff --git a/ext/mixed/js/japanese.js b/ext/mixed/js/japanese.js
index 8b841b2e..ea1c0065 100644
--- a/ext/mixed/js/japanese.js
+++ b/ext/mixed/js/japanese.js
@@ -160,16 +160,17 @@ function jpDistributeFuriganaInflected(expression, reading, source) {
     }
     const offset = source.length - stemLength;
 
-    const stemExpression = source.slice(0, source.length - offset);
-    const stemReading = reading.slice(
-        0, offset === 0 ? reading.length : reading.length - expression.length + stemLength
+    const stemExpression = source.substring(0, source.length - offset);
+    const stemReading = reading.substring(
+        0,
+        offset === 0 ? reading.length : reading.length - expression.length + stemLength
     );
     for (const segment of jpDistributeFurigana(stemExpression, stemReading)) {
         output.push(segment);
     }
 
     if (stemLength !== source.length) {
-        output.push({text: source.slice(stemLength)});
+        output.push({text: source.substring(stemLength)});
     }
 
     return output;