What is the necessity of using declare in TypeScript for declaring variables and functions, and when is it not required?
For instance, why use
declare var foo: number;
when
let foo: number;
seems to achieve the same result (declaring a variable named foo with type number)? What distinguishes them?
When using the declare keyword in TypeScript, it doesn't actually declare a variable but rather informs TypeScript that the variable exists, even if it's not explicitly declared within the code itself. This is useful when working with global variables from other scripts or when combining TypeScript output with external files that declare the variable. Essentially, declare is only for the TypeScript compiler and has no impact on the JavaScript runtime.
If you compile the declaration declare var foo: number; in a TypeScript playground, it will not produce any output for that specific declaration; see this example.
In contrast, using let foo: number; (or var foo: number;) constitutes an actual variable declaration, as demonstrated in this example.
When working with TypeScript, the line "declare var foo: number;" signifies that within the code, there is a variable named foo of type number. Although it may not be explicitly defined, it serves as a form of forward declaration, similar to what is seen in languages like C/C++.
For instance:
Suppose you are writing a TypeScript script that utilizes jQuery. In your HTML page, you would typically include all necessary JavaScript files like this:
<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript" src="myscript.js"></script> <!-- compiled from TypeScript using "tsc myscript.ts" -->
As a result, you do not need to import jQuery directly into your TypeScript code since the generated JavaScript file will already have access to jQuery's functionality. However, the TypeScript transpiler may flag instances of jQuery's signifier "$()" in your code. Since you did not explicitly include jQuery in your TypeScript code (as it was unnecessary), the transpiler does not recognize what "$()" represents and will throw errors related to it. To address these errors, you can declare "var $: JQuery;" at the start of your TypeScript file. This declaration essentially informs the transpiler:
There exists a variable named "$" in the code, which is of type "JQuery". While its definition may not be visible currently, trust that it will be resolved during execution. Pretend for now that there was an explicit "
import "aTSfile.ts";" statement earlier providing a detailed description of "$" as I am explaining it here. Rest assured, JavaScript will understand the true nature of "$" when the script runs. No need to worry about that. ;-)
When using typescript, the declare keyword comes in handy for specifying that a declaration is actually defined elsewhere. To learn more about how to utilize the declare keyword, you can check out this informative link:
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