Since TypeScript extends JavaScript, why is it necessary to write declarations when incorporating JavaScript libraries?

After reviewing some examples provided by developers of TypeScript on GitHub, it seems they utilize stubs for interoperability with JavaScript:

declare module Backbone {
    export class Model {
        constructor (attr? , opts? );
        get(name: string): any;
        set(name: string, val: any): void;
        set(obj: any): void;
        save(attr? , opts? ): void;
        destroy(): void;
        bind(ev: string, f: Function, ctx?: any): void;
        toJSON(): any;
    }
    ...
}
interface JQuery {
    fadeIn(): JQuery;
    fadeOut(): JQuery;
    focus(): JQuery;
    html(): string;
    html(val: string): JQuery;
    show(): JQuery;
    addClass(className: string): JQuery;
    removeClass(className: string): JQuery;
    append(el: HTMLElement): JQuery;
    val(): string;
    val(value: string): JQuery;
    attr(attrName: string): string;
}

It may seem unusual, but can I avoid creating such stubs and simply use the jQuery object directly to call its methods? Considering TypeScript is a superset of JavaScript, one would assume that if something is achievable in JavaScript, it should be doable in TypeScript as well. What are the most effective ways to invoke JavaScript code from TypeScript?

Answer №1

In Javascript, type declarations are necessary as the language is untyped. They need to be added to function and method signatures for static analysis assistance. These declarations do not make the code stubs, but rather provide essential information for proper functionality.

For instance:

The confirm function displays a popup with a query and returns true or false based on user input. In traditional Javascript, you might call it like this:

var x = prompt(1 + 1) + "hello world";

This example is incorrect, as prompt expects a string parameter and returns a boolean value. Typescript addresses this issue by including a declaration in lib.d.ts:

declare function confirm(message?: string): boolean;

If I were to use my previous code without this type declaration, the typescript compiler would detect an error during compilation.

Answer №2

If you choose to omit these declarations, your Typescript code will still be valid. However, including this information about the return type of vanilla Javascript APIs allows Typescript to perform static type checking. Without these declarations, any imports from a plain JS library are assumed to return a value of type any, limiting what Typescript can verify.

This coding practice is referred to as utilizing type declaration files. It's worth noting that you may not even need to create your own declarations, as there are numerous existing declaration files available online for popular Javascript libraries.

Answer №3

One of the advantages of TypeScript is its ability to prevent us from using variables that have not been defined. For instance, consider the following code snippet within a .ts file:

$("#myElement").hide();

If we try to compile this code, TypeScript will generate an error stating that '$' has not been defined. This feature helps us avoid potential mistakes in our code.

When incorporating JavaScript libraries like jQuery into our TypeScript code without declaring the necessary variables (e.g., $), the compiler will alert us about these undefined entities. It's crucial to inform the compiler about external variables to ensure smooth integration.

To address this issue, we can create an ambient definition with the 'any' type:

declare var $: any;

By specifying this ambient declaration, we indicate to the compiler that the variable exists outside the TypeScript environment and can be used without strict typing restrictions. However, it is generally recommended to provide type information for variables to enhance code safety and accuracy.

To define the type of '$', we can leverage existing type definitions using tools like tsd:

tsd install jquery

This command downloads the 'jquery.d.ts' file, which contains the necessary type information for the jQuery library, allowing us to seamlessly integrate it into our application.

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