chore: optimize utilities

.prettierignore

@@ -1,3 +1,2 @@
 *.yml
-dist
 flow-typed

packages/styled-components/src/sheet/GroupedTag.ts

@@ -29,7 +29,7 @@ const DefaultGroupedTag = class DefaultGroupedTag implements GroupedTag {
     return index;
   }
 
-  insertRules(group: number, rules: string | string[]) {
+  insertRules(group: number, rules: string[]) {
     if (group >= this.groupSizes.length) {
       const oldBuffer = this.groupSizes;
       const oldSize = oldBuffer.length;
@@ -53,16 +53,10 @@ const DefaultGroupedTag = class DefaultGroupedTag implements GroupedTag {
 
     let ruleIndex = this.indexOfGroup(group + 1);
 
-    if (Array.isArray(rules)) {
-      for (let i = 0, l = rules.length; i < l; i++) {
-        if (this.tag.insertRule(ruleIndex, rules[i])) {
-          this.groupSizes[group]++;
-          ruleIndex++;
-        }
-      }
-    } else {
-      if (this.tag.insertRule(ruleIndex, rules)) {
+    for (let i = 0, l = rules.length; i < l; i++) {
+      if (this.tag.insertRule(ruleIndex, rules[i])) {
         this.groupSizes[group]++;
+        ruleIndex++;
       }
     }
   }

packages/styled-components/src/sheet/Tag.ts

@@ -20,12 +20,12 @@ export const CSSOMTag = class CSSOMTag implements Tag {
   length: number;
 
   constructor(target?: HTMLElement) {
-    const element = (this.element = makeStyleTag(target));
+    this.element = makeStyleTag(target);
 
     // Avoid Edge bug where empty style elements don't create sheets
-    element.appendChild(document.createTextNode(''));
+    this.element.appendChild(document.createTextNode(''));
 
-    this.sheet = getSheet(element);
+    this.sheet = getSheet(this.element);
     this.length = 0;
   }
 
@@ -46,8 +46,9 @@ export const CSSOMTag = class CSSOMTag implements Tag {
 
   getRule(index: number): string {
     const rule = this.sheet.cssRules[index];
+
     // Avoid IE11 quirk where cssText is inaccessible on some invalid rules
-    if (rule !== undefined && typeof rule.cssText === 'string') {
+    if (rule && rule.cssText) {
       return rule.cssText;
     } else {
       return '';
@@ -62,8 +63,8 @@ export const TextTag = class TextTag implements Tag {
   length: number;
 
   constructor(target?: HTMLElement) {
-    const element = (this.element = makeStyleTag(target));
-    this.nodes = element.childNodes;
+    this.element = makeStyleTag(target);
+    this.nodes = this.element.childNodes;
     this.length = 0;
   }
 

packages/styled-components/src/utils/isPlainObject.ts

@@ -1,12 +1,9 @@
-export default function isPlainObject(x: any): boolean {
+export default function isPlainObject(x: any): x is Record<any, any> {
   return (
     x !== null &&
     typeof x === 'object' &&
-    /* a check for empty prototype would be more typical, but that
-       doesn't play well with objects created in different vm contexts */
-    (!x.constructor || x.constructor.name === 'Object') &&
-    (x.toString ? x.toString() : Object.prototype.toString.call(x)) === '[object Object]' &&
+    x.constructor.name === Object.name &&
     /* check for reasonable markers that the object isn't an element for react & preact/compat */
-    !('props' in x && (x.$$typeof || x.constructor === undefined))
+    !('props' in x && x.$$typeof)
   );
 }