performance: Use startsWith in many places.

Using startsWith is faster than indexOf, especially for long strings
and short prefixes.  It's also a lot more readable.  The only reason
we weren't using it was when a lot of the code was originally written,
it wasn't available.

static/js/common.js

@@ -71,13 +71,13 @@ exports.phrase_match = function (query, phrase) {
     query = query.toLowerCase();
 
     phrase = phrase.toLowerCase();
-    if (phrase.indexOf(query) === 0) {
+    if (phrase.startsWith(query)) {
         return true;
     }
 
     const parts = phrase.split(' ');
     for (i = 0; i < parts.length; i += 1) {
-        if (parts[i].indexOf(query) === 0) {
+        if (parts[i].startsWith(query)) {
             return true;
         }
     }

static/js/composebox_typeahead.js

@@ -958,7 +958,7 @@ exports.initialize = function () {
             // The matcher for "stream" is strictly prefix-based,
             // because we want to avoid mixing up streams.
             const q = this.query.trim().toLowerCase();
-            return item.toLowerCase().indexOf(q) === 0;
+            return item.toLowerCase().startsWith(q);
         },
     });
 

static/js/markdown.js

@@ -170,7 +170,7 @@ exports.add_topic_links = function (message) {
 };
 
 exports.is_status_message = function (raw_content) {
-    return raw_content.indexOf('/me ') === 0;
+    return raw_content.startsWith('/me ');
 };
 
 function make_emoji_span(codepoint, title, alt_text) {
@@ -251,7 +251,7 @@ function handleTex(tex, fullmatch) {
     try {
         return katex.renderToString(tex);
     } catch (ex) {
-        if (ex.message.indexOf('KaTeX parse error') === 0) { // TeX syntax error
+        if (ex.message.startsWith('KaTeX parse error')) { // TeX syntax error
             return '<span class="tex-error">' + escape(fullmatch) + '</span>';
         }
         blueslip.error(ex);

static/js/people.js

@@ -824,7 +824,7 @@ exports.build_termlet_matcher = function (termlet) {
         const names = full_name.toLowerCase().split(' ');
 
         return _.any(names, function (name) {
-            if (name.indexOf(termlet) === 0) {
+            if (name.startsWith(termlet)) {
                 return true;
             }
         });
@@ -840,7 +840,7 @@ exports.build_person_matcher = function (query) {
     return function (user) {
         const email = user.email.toLowerCase();
 
-        if (email.indexOf(query) === 0) {
+        if (email.startsWith(query)) {
             return true;
         }
 

static/js/search_suggestion.js

@@ -415,10 +415,11 @@ function get_special_filter_suggestions(last, operators, suggestions) {
         // returns the substring after the ":" symbol.
         const suggestion_operand = s.search_string.substring(s.search_string.indexOf(":") + 1);
         // e.g for `att` search query, `has:attachment` should be suggested.
-        const show_operator_suggestions = last.operator === 'search' && suggestion_operand.toLowerCase().indexOf(last_string) === 0;
-        return s.search_string.toLowerCase().indexOf(last_string) === 0 ||
+        const show_operator_suggestions = last.operator === 'search' &&
+            suggestion_operand.toLowerCase().startsWith(last_string);
+        return s.search_string.toLowerCase().startsWith(last_string) ||
                show_operator_suggestions ||
-               s.description.toLowerCase().indexOf(last_string) === 0;
+               s.description.toLowerCase().startsWith(last_string);
     });
 
     // Only show home if there's an empty bar
@@ -541,15 +542,20 @@ function get_sent_by_me_suggestions(last, operators) {
         return [];
     }
 
-    if (last.operator === '' || sender_query.indexOf(last_string) === 0 ||
-        sender_me_query.indexOf(last_string) === 0 || last_string === sent_string) {
+    if (last.operator === '' ||
+        sender_query.startsWith(last_string) ||
+        sender_me_query.startsWith(last_string) ||
+        last_string === sent_string
+    ) {
         return [
             {
                 search_string: sender_query,
                 description: description,
             },
         ];
-    } else if (from_query.indexOf(last_string) === 0 || from_me_query.indexOf(last_string) === 0) {
+    } else if (from_query.startsWith(last_string) ||
+        from_me_query.startsWith(last_string)
+    ) {
         return [
             {
                 search_string: from_query,
@@ -567,7 +573,7 @@ function get_operator_suggestions(last) {
     let last_operand = last.operand;
 
     let negated = false;
-    if (last_operand.indexOf("-") === 0) {
+    if (last_operand.startsWith("-")) {
         negated = true;
         last_operand = last_operand.slice(1);
     }

static/js/stream_sort.js

@@ -25,7 +25,7 @@ function filter_streams_by_search(streams, search_term) {
             const cands = lower_stream_name.split(" ");
             cands.push(lower_stream_name);
             return _.any(cands, function (name) {
-                return name.indexOf(search_term) === 0;
+                return name.startsWith(search_term);
             });
         });
     });

static/js/upload.js

@@ -1,5 +1,5 @@
 function make_upload_absolute(uri) {
-    if (uri.indexOf(compose.uploads_path) === 0) {
+    if (uri.startsWith(compose.uploads_path)) {
         // Rewrite the URI to a usable link
         return compose.uploads_domain + uri;
     }

static/shared/js/typeahead.js

@@ -30,7 +30,7 @@ function query_matches_string(query, source_str, split_char) {
         if (sources[i] === undefined) {
             return false;
         }
-        return sources[i].indexOf(queries[i]) === 0;
+        return sources[i].startsWith(queries[i]);
     }
 
     // For a single token, the match can be anywhere in the string.