TypeScript常见类型及应用示例讲解

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);
// Argument of type 'number' is not assignable to parameter of type 'string'.

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());
  // 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());
  // 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'.
// Type '{ myID: number; }' is not assignable to type '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);
}