Why do you need the `declare` keyword in TypeScript? Explain with examples

Hi everyone! I started learning TypeScript and came across the `declare` keyword. The official documentation says it is used for "declaring variables, functions, and classes that exist somewhere else." But I don't quite understand why it's needed and how it works in practice.\n\nFor example, I'm writing a simple script that uses the global variable `$` (jQuery). I know it's loaded via CDN in HTML, but TypeScript complains that `$` is not defined. I tried writing:\n\n```typescript\ndeclare var $: any;\n```\n\nAnd the error disappeared. But why can't I just write `var $: any;`? What's the difference? If I write `let $: any;`, that also works, but then TypeScript will think I'm creating a new variable, not describing an existing one.\n\nI also saw that `declar