diff --git a/app/assets/javascripts/build.js b/app/assets/javascripts/build.js
index 8fa1aceddffa725e135224d0eed0be2b736ad72e..c0dc7a79b0c4a535d0762b0aab5f888d72ce65b9 100644
--- a/app/assets/javascripts/build.js
+++ b/app/assets/javascripts/build.js
@@ -86,7 +86,7 @@
           if (window.location.hash === DOWN_BUILD_TRACE) {
             $("html,body").scrollTop(this.$buildTrace.height());
           }
-          if (removeRefreshStatuses.indexOf(buildData.status) >= 0) {
+          if (removeRefreshStatuses.indexOf(buildData.status) !== -1) {
             this.$buildRefreshAnimation.remove();
             return this.initScrollMonitor();
           }
diff --git a/app/assets/javascripts/gl_dropdown.js b/app/assets/javascripts/gl_dropdown.js
index a01662e2f9ef76c65de6f26506eba9da8161449c..9e6ed06054bb037dcd8a6c914b07ef0336e19ca1 100644
--- a/app/assets/javascripts/gl_dropdown.js
+++ b/app/assets/javascripts/gl_dropdown.js
@@ -63,7 +63,7 @@
     }
 
     GitLabDropdownFilter.prototype.shouldBlur = function(keyCode) {
-      return BLUR_KEYCODES.indexOf(keyCode) >= 0;
+      return BLUR_KEYCODES.indexOf(keyCode) !== -1;
     };
 
     GitLabDropdownFilter.prototype.filter = function(search_text) {
@@ -605,7 +605,7 @@
       var occurrences;
       occurrences = fuzzaldrinPlus.match(text, term);
       return text.split('').map(function(character, i) {
-        if (indexOf.call(occurrences, i) >= 0) {
+        if (indexOf.call(occurrences, i) !== -1) {
           return "<b>" + character + "</b>";
         } else {
           return character;
@@ -748,7 +748,7 @@
         return function(e) {
           var $listItems, PREV_INDEX, currentKeyCode;
           currentKeyCode = e.which;
-          if (ARROW_KEY_CODES.indexOf(currentKeyCode) >= 0) {
+          if (ARROW_KEY_CODES.indexOf(currentKeyCode) !== -1) {
             e.preventDefault();
             e.stopImmediatePropagation();
             PREV_INDEX = currentIndex;
diff --git a/app/assets/javascripts/issuable.js.es6 b/app/assets/javascripts/issuable.js.es6
index 8df86f682183bf54c47a72b957fe330ff01d7479..3bfce32768a6923255a1809d3163447fbdce91fe 100644
--- a/app/assets/javascripts/issuable.js.es6
+++ b/app/assets/javascripts/issuable.js.es6
@@ -116,7 +116,7 @@
         formData = $.param(formData);
         formAction = form.attr('action');
         issuesUrl = formAction;
-        issuesUrl += "" + (formAction.indexOf('?') < 0 ? '?' : '&');
+        issuesUrl += "" + (formAction.indexOf('?') === -1 ? '?' : '&');
         issuesUrl += formData;
         return gl.utils.visitUrl(issuesUrl);
       };
diff --git a/app/assets/javascripts/merge_request_widget.js.es6 b/app/assets/javascripts/merge_request_widget.js.es6
index 88f08bbaa345ee6cafdf524a569a64308b31a804..00c6c05061219b6985b7152ae77869ef092f52ec 100644
--- a/app/assets/javascripts/merge_request_widget.js.es6
+++ b/app/assets/javascripts/merge_request_widget.js.es6
@@ -83,7 +83,7 @@ require('./smart_interval');
         return function() {
           var page;
           page = $('body').data('page').split(':').last();
-          if (allowedPages.indexOf(page) < 0) {
+          if (allowedPages.indexOf(page) === -1) {
             return _this.clearEventListeners();
           }
         };
@@ -233,7 +233,7 @@ require('./smart_interval');
       }
       $('.ci_widget').hide();
       allowed_states = ["failed", "canceled", "running", "pending", "success", "success_with_warnings", "skipped", "not_found"];
-      if (indexOf.call(allowed_states, state) >= 0) {
+      if (indexOf.call(allowed_states, state) !== -1) {
         $('.ci_widget.ci-' + state).show();
         switch (state) {
           case "failed":
diff --git a/app/assets/javascripts/new_branch_form.js b/app/assets/javascripts/new_branch_form.js
index 3f678b93f73115eecde68168b562ddd8c0a5be3d..5828f460a235a1f9a25a9019fdf26cb0361ee5b5 100644
--- a/app/assets/javascripts/new_branch_form.js
+++ b/app/assets/javascripts/new_branch_form.js
@@ -81,7 +81,7 @@
       var errorMessage, errors, formatter, unique, validator;
       this.branchNameError.empty();
       unique = function(values, value) {
-        if (indexOf.call(values, value) < 0) {
+        if (indexOf.call(values, value) === -1) {
           values.push(value);
         }
         return values;
diff --git a/app/assets/javascripts/project.js b/app/assets/javascripts/project.js
index 7c03c8b72d4b15a11ee6e2642b9af2b711712706..db7ceaa2421d826e70142e58bb5bb6c3edc156f0 100644
--- a/app/assets/javascripts/project.js
+++ b/app/assets/javascripts/project.js
@@ -116,7 +116,7 @@
             if ($('input[name="ref"]').length) {
               var $form = $dropdown.closest('form');
               var action = $form.attr('action');
-              var divider = action.indexOf('?') < 0 ? '?' : '&';
+              var divider = action.indexOf('?') === -1 ? '?' : '&';
               gl.utils.visitUrl(action + '' + divider + '' + $form.serialize());
             }
           }
diff --git a/changelogs/unreleased/28723-consistent-handling-indexof.yml b/changelogs/unreleased/28723-consistent-handling-indexof.yml
new file mode 100644
index 0000000000000000000000000000000000000000..95d6181d5fa115972ba1af48e141aa13cfe8f034
--- /dev/null
+++ b/changelogs/unreleased/28723-consistent-handling-indexof.yml
@@ -0,0 +1,4 @@
+---
+title: Keep consistent in handling indexOf results
+merge_request: 9531
+author: Takuya Noguchi