ts declare使用

In TypeScript, the `declare` keyword is used to tell the compiler that a variable, function, class, or namespace already exists and is defined elsewhere. This is useful when you want to use a library or framework that was written in JavaScript, but you want to write your code in TypeScript.

Here are a few examples of how to use `declare` in TypeScript:

1. Declare a global variable:

declare const myVar: string;

This tells TypeScript that `myVar` is a global variable of type `string`, but the variable is not defined in the current file.

2. Declare a function:

declare function myFunc(param: string): number;

This tells TypeScript that `myFunc` is a function that takes a parameter of type `string` and returns a value of type `number`, but the function is not defined in the current file.

3. Declare a class:

declare class MyClass {
  constructor(param: string);
  myMethod(): void;
}

This tells TypeScript that `MyClass` is a class with a constructor that takes a parameter of type `string`, and the class has a method called `myMethod` that returns `void`, but the class is not defined in the current file.

4. Declare a namespace:

declare namespace MyNamespace {
  const myVar: string;
  function myFunc(param: string): number;
  class MyClass {
    constructor(param: string);
    myMethod(): void;
  }
}

This tells TypeScript that `MyNamespace` is a namespace that contains a variable called `myVar`, a function called `myFunc`, and a class called `MyClass`, but the namespace is not defined in the current file.

Using `declare` in TypeScript is a powerful tool for working with existing JavaScript code and libraries.