Merge branch 'master' into dl0312

Author: simsim0709

content/docs/code-splitting.md

@@ -225,7 +225,7 @@ const App = () => (
 
 ## Named Exports {#named-exports}
 
-`React.lazy`는 현재 default exports만 지원합니다. named exports를 사용하고자 한다면 default로 이름을 재정의한 중간 모듈을 생성할 수 있습니다. 이렇게 하면 treeshaking이 계속 동작하고 사용하지 않는 컴포넌트는 가져오지 않습니다.
+`React.lazy`는 현재 default exports만 지원합니다. named exports를 사용하고자 한다면 default로 이름을 재정의한 중간 모듈을 생성할 수 있습니다. 이렇게 하면 tree shaking이 계속 동작하고 사용하지 않는 컴포넌트는 가져오지 않습니다.
 
 ```js
 // ManyComponents.js

content/docs/hooks-effect.md

@@ -473,7 +473,7 @@ In the future, the second argument might get added automatically by a build-time
 >
 >If you use this optimization, make sure the array includes **all values from the component scope (such as props and state) that change over time and that are used by the effect**. Otherwise, your code will reference stale values from previous renders. Learn more about [how to deal with functions](/docs/hooks-faq.html#is-it-safe-to-omit-functions-from-the-list-of-dependencies) and [what to do when the array changes too often](/docs/hooks-faq.html#what-can-i-do-if-my-effect-dependencies-change-too-often).
 >
->If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array (`[]`) as a second argument. This tells React that your effect doesn't depend on *any* values from props or state, so it never needs to re-run. This isn't handled as a special case -- it follows directly from how the inputs array always works.
+>If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array (`[]`) as a second argument. This tells React that your effect doesn't depend on *any* values from props or state, so it never needs to re-run. This isn't handled as a special case -- it follows directly from how the dependencies array always works.
 >
 >If you pass an empty array (`[]`), the props and state inside the effect will always have their initial values. While passing `[]` as the second argument is closer to the familiar `componentDidMount` and `componentWillUnmount` mental model, there are usually [better](/docs/hooks-faq.html#is-it-safe-to-omit-functions-from-the-list-of-dependencies) [solutions](/docs/hooks-faq.html#what-can-i-do-if-my-effect-dependencies-change-too-often) to avoid re-running effects too often. Also, don't forget that React defers running `useEffect` until after the browser has painted, so doing extra work is less of a problem.
 >

content/docs/hooks-faq.md

@@ -740,7 +740,7 @@ The [`useMemo`](/docs/hooks-reference.html#usememo) Hook lets you cache calculat
 const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);
 ```
 
-This code calls `computeExpensiveValue(a, b)`. But if the inputs `[a, b]` haven't changed since the last value, `useMemo` skips calling it a second time and simply reuses the last value it returned.
+This code calls `computeExpensiveValue(a, b)`. But if the dependencies `[a, b]` haven't changed since the last value, `useMemo` skips calling it a second time and simply reuses the last value it returned.
 
 Remember that the function passed to `useMemo` runs during rendering. Don't do anything there that you wouldn't normally do while rendering. For example, side effects belong in `useEffect`, not `useMemo`.
 
@@ -767,7 +767,7 @@ Note that this approach won't work in a loop because Hook calls [can't](/docs/ho
 
 ### How to create expensive objects lazily? {#how-to-create-expensive-objects-lazily}
 
-`useMemo` lets you [memoize an expensive calculation](#how-to-memoize-calculations) if the inputs are the same. However, it only serves as a hint, and doesn't *guarantee* the computation won't re-run. But sometimes you need to be sure an object is only created once.
+`useMemo` lets you [memoize an expensive calculation](#how-to-memoize-calculations) if the dependencies are the same. However, it only serves as a hint, and doesn't *guarantee* the computation won't re-run. But sometimes you need to be sure an object is only created once.
 
 **The first common use case is when creating the initial state is expensive:**
 
@@ -809,13 +809,10 @@ function Image(props) {
 
   // ✅ IntersectionObserver is created lazily once
   function getObserver() {
-    let observer = ref.current;
-    if (observer !== null) {
-      return observer;
+    if (ref.current === null) {
+      ref.current = new IntersectionObserver(onIntersect);
     }
-    let newObserver = new IntersectionObserver(onIntersect);
-    ref.current = newObserver;
-    return newObserver;
+    return ref.current;
   }
 
   // When you need it, call getObserver()

content/docs/reference-react.md

@@ -19,7 +19,7 @@ redirect_from:
 
 ### 컴포넌트 {#components}
 
-React 컴포넌트를 사용하면 UI를 독립적이고 재사용할 수 있는 부분으로 나누고 각 부분을 분리하여 생각할 수 있습니다. React 컴포넌트는 `React.Component` 또는 `React.PureComponent`로 세부적으로 나누어 정의할 수 있습니다.
+React 컴포넌트를 사용하면 UI를 독립적이고 재사용할 수 있는 부분으로 나누고 각 부분을 분리하여 생각할 수 있습니다. React 컴포넌트는 `React.Component` 또는 `React.PureComponent`로 세부적으로 나누어 정의할 수 있습니다.
 
 - [`React.Component`](#reactcomponent)
 - [`React.PureComponent`](#reactpurecomponent)