Add formatter cache (i18next#1843)

* Add formatter cache

* Fix unnamed function error

Author: pwolaq

src/Formatter.js

@@ -39,27 +39,45 @@ function parseFormatStr(formatStr) {
   };
 }
 
+function createCachedFormatter(fn) {
+  const cache = Object.create(null);
+  return function invokeFormatter(val, lng, options) {
+    const key = lng + JSON.stringify(options);
+    let formatter = cache[key];
+    if (!formatter) {
+      formatter = fn(lng, options);
+      cache[key] = formatter;
+    }
+    return formatter(val);
+  };
+}
+
 class Formatter {
   constructor(options = {}) {
     this.logger = baseLogger.create('formatter');
 
     this.options = options;
     this.formats = {
-      number: (val, lng, options) => {
-        return new Intl.NumberFormat(lng, options).format(val);
-      },
-      currency: (val, lng, options) => {
-        return new Intl.NumberFormat(lng, { ...options, style: 'currency' }).format(val);
-      },
-      datetime: (val, lng, options) => {
-        return new Intl.DateTimeFormat(lng, { ...options }).format(val);
-      },
-      relativetime: (val, lng, options) => {
-        return new Intl.RelativeTimeFormat(lng, { ...options }).format(val, options.range || 'day');
-      },
-      list: (val, lng, options) => {
-        return new Intl.ListFormat(lng, { ...options }).format(val);
-      },
+      number: createCachedFormatter((lng, options) => {
+        const formatter = new Intl.NumberFormat(lng, options);
+        return (val) => formatter.format(val);
+      }),
+      currency: createCachedFormatter((lng, options) => {
+        const formatter = new Intl.NumberFormat(lng, { ...options, style: 'currency' });
+        return (val) => formatter.format(val);
+      }),
+      datetime: createCachedFormatter((lng, options) => {
+        const formatter = new Intl.DateTimeFormat(lng, { ...options });
+        return (val) => formatter.format(val);
+      }),
+      relativetime: createCachedFormatter((lng, options) => {
+        const formatter = new Intl.RelativeTimeFormat(lng, { ...options });
+        return (val) => formatter.format(val, options.range || 'day');
+      }),
+      list: createCachedFormatter((lng, options) => {
+        const formatter = new Intl.ListFormat(lng, { ...options });
+        return (val) => formatter.format(val);
+      }),
     };
     this.init(options);
   }