深入了解TypeScript中常用类型的使用_javascript技巧_

let obj: any = { x: 0 }; // None of the following lines of code will throw compiler errors. // Using `any` disables all further type checking, and it is assumed // you know the environment better than TypeScript. obj.foo(); obj(); obj.bar = 100; obj = "hello"; const n: number = obj;

let myName: string = "Alice";

// No type annotation needed -- 'myName' inferred as type 'string' let myName = "Alice";

// Parameter type annotation function greet(name: string) { console.log("Hello, " + name.toUpperCase() + "!!"); }

// Would be a runtime error if executed! greet(42);

function getFavoriteNumber(): number { return 26; }

// No type annotations here, but TypeScript can spot the bug const names = ["Alice", "Bob", "Eve"]; // Contextual typing for function names.forEach(function (s) { console.log(s.toUppercase()); // 被string[]类型的数组调用，自动推导为string类型 // Property 'toUppercase' does not exist on type 'string'. Did you mean 'toUpperCase'? }); // Contextual typing also applies to arrow functions names.forEach((s) => { console.log(s.toUppercase()); // 被string[]类型的数组调用，自动推导为string类型 // Property 'toUppercase' does not exist on type 'string'. Did you mean 'toUpperCase'? });

// The parameter's type annotation is an object type function printCoord(pt: { x: number; y: number }) { console.log("The coordinate's x value is " + pt.x); console.log("The coordinate's y value is " + pt.y); } printCoord({ x: 3, y: 7 });

function printName(obj: { first: string; last?: string }) { // ... } // Both OK printName({ first: "Bob" }); printName({ first: "Alice", last: "Alisson" });

function printName(obj: { first: string; last?: string }) { // Error - might crash if 'obj.last' wasn't provided! console.log(obj.last.toUpperCase()); //Object is possibly 'undefined'. if (obj.last !== undefined) { // OK console.log(obj.last.toUpperCase()); } // A safe alternative using modern JavaScript syntax: console.log(obj.last?.toUpperCase()); }

function printId(id: number | string) { console.log("Your ID is: " + id); } // OK printId(101); // OK printId("202"); // Error printId({ myID: 22342 }); // Argument of type '{ myID: number; }' is not assignable to parameter of type 'string | number'.

function printId(id: number | string) { console.log(id.toUpperCase()); // Property 'toUpperCase' does not exist on type 'string | number'. // Property 'toUpperCase' does not exist on type 'number'. }

function printId(id: number | string) { if (typeof id === "string") { // In this branch, id is of type 'string' console.log(id.toUpperCase()); } else { // Here, id is of type 'number' console.log(id); } }

function welcomePeople(x: string[] | string) { if (Array.isArray(x)) { // Here: 'x' is 'string[]' console.log("Hello, " + x.join(" and ")); } else { // Here: 'x' is 'string' console.log("Welcome lone traveler " + x); } }

// Return type is inferred as number[] | string function getFirstThree(x: number[] | string) { return x.slice(0, 3); }

type Point = { x: number; y: number; }; // Exactly the same as the earlier example function printCoord(pt: Point) { console.log("The coordinate's x value is " + pt.x); console.log("The coordinate's y value is " + pt.y); } printCoord({ x: 100, y: 100 });

type ID = number | strin
                
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