];constloggedInUser = users.find((u) =>u.name=== loggedInUsername);console.log(loggedInUser.age);// 报错:“loggedInUser”可能为“未定义”。 示例二: 当strictNullChecks: true 时 typeArray= {find(predicate:(value:any, index:number) =>boolean): S |undefined; }; 上述示例中,find 方法...
interfaceAdmin{name:string;privileges:string[];}interfaceEmployee{name:string;startDate:Date;}type UnknownEmployee=Employee|Admin;functionprintEmployeeInformation(emp:UnknownEmployee){console.log("Name: "+emp.name);if("privileges"inemp){console.log("Privileges: "+emp.privileges);}if("startDate"inemp...
* value around similar to how you’d use instance fields in classes. * * Usage note: if you need the result of useRef to be directly mutable, include `| null` in the type * of the generic argument. * * @version 16.8.0 * @see https://reactjs.org/docs/hooks-reference.html#useref...
TypeScript 是一种由微软开发的自由和开源的编程语言。它是 JavaScript 的一个超集,而且本质上向这个语言添加了可选的静态类型和基于类的面向对象编程。
(mod){if(mod&&mod.__esModule)returnmod;varresult={};if(mod!=null)for(varkinmod)if(Object.hasOwnProperty.call(mod,k))result[k]=mod[k];result["default"]=mod;returnresult;};Object.defineProperty(exports,"__esModule",{value:true});constpath=__importStar(require("path"));console.log(...
let value: true | false; let value: boolean; (4)模板字面量类型 在TypeScript 4.1 版本中新增了模板字面量类型。什么是模板字面量类型呢?它一字符串字面量类型为基础,可以通过联合类型扩展成多个字符串。它与 JavaScript 的模板字符串语法相同,但是只能用在类型定义中使用。
Property '1' is missing in type '[string]' but required in type '[string, boolean]'. 2.9Void 类型 某种程度上来说,void 类型像是与 any 类型相反,它表示没有任何类型。当一个函数没有返回值时,你通常会见到其返回值类型是 void: // 声明函数返回值为void ...
Bug Type: TypeScript Environment Vue Version: 3.4.38 Element Plus Version: 2.8.1 Browser / OS: chrome Build Tool: Vite Reproduction Related Component All Reproduction Link Element Plus Playground Steps to reproduce {{ label }} What is Ex...
2.2. Example: Basic Usage of Default Parameters In the following example, the functionfullName()specifies the default value the last parametermiddleNameas an empty string. functionfullName(firstName:String,lastName:String,middleName:String=""):string{return`${firstName}${middleName}${lastName}`...
type State = {text: string;};class App extends React.Component<Props, State> {state = {text: "",};// 在 = 的右侧输入onChange = (e: React.FormEvent<HTMLInputElement>): void => {this.setState({ text: e.currentTarget.value });};render() {return (<div><input type="text" value...