refactor: native Array.flat() (#4806)

- Node 12 feature
- remove misleading "old syntax"
  * "old syntax" implies deprecation,
    but in this case, it's just different syntax

lib/plugins/helper/css.js

@@ -3,31 +3,18 @@
 const { htmlTag, url_for } = require('hexo-util');
 const { default: moize } = require('moize');
 
-const flatten = function(arr, result = []) {
-  for (const i in arr) {
-    const value = arr[i];
-    if (Array.isArray(value)) {
-      flatten(value, result);
-    } else {
-      result.push(value);
-    }
-  }
-  return result;
-};
-
 function cssHelper(...args) {
   let result = '\n';
 
-  flatten(args).forEach(item => {
-    // Old syntax
+  args.flat(Infinity).forEach(item => {
     if (typeof item === 'string' || item instanceof String) {
       let path = item;
       if (!path.endsWith('.css')) {
         path += '.css';
       }
       result += `<link rel="stylesheet" href="${url_for.call(this, path)}">\n`;
     } else {
-      // New syntax
+      // Custom attributes
       item.href = url_for.call(this, item.href);
       if (!item.href.endsWith('.css')) item.href += '.css';
       result += htmlTag('link', { rel: 'stylesheet', ...item }) + '\n';

lib/plugins/helper/js.js

@@ -3,33 +3,18 @@
 const { htmlTag, url_for } = require('hexo-util');
 const { default: moize } = require('moize');
 
-/* flatten() to be replaced by Array.flat()
-after Node 10 has reached EOL */
-const flatten = function(arr, result = []) {
-  for (const i in arr) {
-    const value = arr[i];
-    if (Array.isArray(value)) {
-      flatten(value, result);
-    } else {
-      result.push(value);
-    }
-  }
-  return result;
-};
-
 function jsHelper(...args) {
   let result = '\n';
 
-  flatten(args).forEach(item => {
-    // Old syntax
+  args.flat(Infinity).forEach(item => {
     if (typeof item === 'string' || item instanceof String) {
       let path = item;
       if (!path.endsWith('.js')) {
         path += '.js';
       }
       result += `<script src="${url_for.call(this, path)}"></script>\n`;
     } else {
-      // New syntax
+      // Custom attributes
       item.src = url_for.call(this, item.src);
       if (!item.src.endsWith('.js')) item.src += '.js';
       result += htmlTag('script', { ...item }, '') + '\n';