# 通過 PropTypes 進行類型檢查
隨著 App 的增長，你可能或捕獲很多關于 類型檢查的 bugs 。對于一些應用，你可以使用 JavaScript 擴展比如 [Flow](https://flowtype.org/) 或者 [TypeScript](https://www.typescriptlang.org/) 類對整個應用進行類型檢查。但是即使你不用這些，React 也有一些內置的類型檢查能力。要在一個組件的 props 上運行類型檢查，可以分配特定的 propTypes 屬性：
~~~
class Greeting extends React.Component {
render() {
return (
<h1>Hello, {this.props.name}</h1>
);
}
}
Greeting.propTypes = {
name: React.PropTypes.string
};
~~~
React.PropTypes 暴露了一系列的驗證器，可以用來確保接受的數據是有效的。在這個例子中，我們使用 React.PropTypes.string。當一個無效的值提供為一個 prop，在 JavaScript 控制臺中將會顯示一個警告。處于性能原因，propTypes只在開發模式中進行檢查。
## React.PropTypes
這里是一個示例記錄提供的不同的驗證器：
~~~
MyComponent.propTypes = {
// You can declare that a prop is a specific JS primitive. By default, these
// are all optional.
optionalArray: React.PropTypes.array,
optionalBool: React.PropTypes.bool,
optionalFunc: React.PropTypes.func,
optionalNumber: React.PropTypes.number,
optionalObject: React.PropTypes.object,
optionalString: React.PropTypes.string,
optionalSymbol: React.PropTypes.symbol,
// Anything that can be rendered: numbers, strings, elements or an array
// (or fragment) containing these types.
optionalNode: React.PropTypes.node,
// A React element.
optionalElement: React.PropTypes.element,
// You can also declare that a prop is an instance of a class. This uses
// JS's instanceof operator.
optionalMessage: React.PropTypes.instanceOf(Message),
// You can ensure that your prop is limited to specific values by treating
// it as an enum.
optionalEnum: React.PropTypes.oneOf(['News', 'Photos']),
// An object that could be one of many types
optionalUnion: React.PropTypes.oneOfType([
React.PropTypes.string,
React.PropTypes.number,
React.PropTypes.instanceOf(Message)
]),
// An array of a certain type
optionalArrayOf: React.PropTypes.arrayOf(React.PropTypes.number),
// An object with property values of a certain type
optionalObjectOf: React.PropTypes.objectOf(React.PropTypes.number),
// An object taking on a particular shape
optionalObjectWithShape: React.PropTypes.shape({
color: React.PropTypes.string,
fontSize: React.PropTypes.number
}),
// You can chain any of the above with `isRequired` to make sure a warning
// is shown if the prop isn't provided.
requiredFunc: React.PropTypes.func.isRequired,
// A value of any data type
requiredAny: React.PropTypes.any.isRequired,
// You can also specify a custom validator. It should return an Error
// object if the validation fails. Don't `console.warn` or throw, as this
// won't work inside `oneOfType`.
customProp: function(props, propName, componentName) {
if (!/matchme/.test(props[propName])) {
return new Error(
'Invalid prop `' + propName + '` supplied to' +
' `' + componentName + '`. Validation failed.'
);
}
},
// You can also supply a custom validator to `arrayOf` and `objectOf`.
// It should return an Error object if the validation fails. The validator
// will be called for each key in the array or object. The first two
// arguments of the validator are the array or object itself, and the
// current item's key.
customArrayProp: React.PropTypes.arrayOf(function(propValue, key, componentName, location, propFullName) {
if (!/matchme/.test(propValue[key])) {
return new Error(
'Invalid prop `' + propFullName + '` supplied to' +
' `' + componentName + '`. Validation failed.'
);
}
})
};
~~~
## 要求單獨的 Children
With React.PropTypes.element you can specify that only a single child can be passed to a component as children.
通過 React.PropTypes.element 你可以指定只有一個單獨的 child 可以傳遞到一個組件作為 children。
~~~
class MyComponent extends React.Component {
render() {
// This must be exactly one element or it will warn.
const children = this.props.children;
return (
<div>
{children}
</div>
);
}
}
MyComponent.propTypes = {
children: React.PropTypes.element.isRequired
};
~~~
## 默認的 prop 值
可以定義你的 props 默認值，通過賦值到特定的 defaultProps 屬性即可：
~~~
class Greeting extends React.Component {
render() {
return (
<h1>Hello, {this.props.name}</h1>
);
}
}
// Specifies the default values for props:
Greeting.defaultProps = {
name: 'Stranger'
};
// Renders "Hello, Stranger":
ReactDOM.render(
<Greeting />,
document.getElementById('example')
);
~~~
defaultProps 被用于確保 this.props.name 將有一個值，即使它沒有被父組件指定。propTypes 類型檢查發生在 defaultProps 被解析之后，所以類型檢查也可以應用到 defaultProps 上。
