Add more information on declare keyword (#117)

* add defition for declare

* update books

* update website

Author: gibbok

README-zh_CN.md

@@ -1140,6 +1140,8 @@ npm install --save-dev @types/library-name
 
 即使在 JavaScript 文件中，您也可以通过 `// @ts-check` 使用环境声明。
 
+`declare` 关键字可以为现有的 JavaScript 代码启用类型定义，而无需导入它，作为来自另一个文件或全局的类型的占位符。
+
 ### 属性检测和多余属性检测
 
 TypeScript 基于结构类型系统，但过多的属性检查是 TypeScript 的一个属性，它允许它检查对象是否具有类型中指定的确切属性。

README.md

@@ -1145,6 +1145,8 @@ For your defined Ambient Declarations, you can import using the "triple-slash" r
 
 You can use Ambient Declarations even within JavaScript files using `// @ts-check`.
 
+The `declare` keyword enables type definitions for existing JavaScript code without importing it, serving as a placeholder for types from another file or globally.
+
 ### Property Checking and Excess Property Checking
 
 TypeScript is based on a structural type system but excess property checking is a property of TypeScript which allows it to check whether an object has the exact properties specified in the type.

downloads/typescript-book-zh_CN.epub

@@ -503,6 +503,8 @@ For your defined Ambient Declarations, you can import using the "triple-slash" r
 
 You can use Ambient Declarations even within JavaScript files using `// @ts-check`.
 
+The `declare` keyword enables type definitions for existing JavaScript code without importing it, serving as a placeholder for types from another file or globally.
+
 ### Property Checking and Excess Property Checking
 
 TypeScript is based on a structural type system but excess property checking is a property of TypeScript which allows it to check whether an object has the exact properties specified in the type.

downloads/typescript-book-zh_CN.pdf

@@ -506,6 +506,8 @@ npm install --save-dev @types/library-name
 
 即使在 JavaScript 文件中，您也可以通过 `// @ts-check` 使用环境声明。
 
+`declare` 关键字可以为现有的 JavaScript 代码启用类型定义，而无需导入它，作为来自另一个文件或全局的类型的占位符。
+
 ### 属性检测和多余属性检测
 
 TypeScript 基于结构类型系统，但过多的属性检查是 TypeScript 的一个属性，它允许它检查对象是否具有类型中指定的确切属性。