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### 1. What "superset" means

TypeScript **extends** the JavaScript language:

- adds **static typing** (type annotations, interfaces, generics);
- introduces **new syntax** (for example, `enum`, `type`, `interface`);
- supports **future JS features** that are not yet implemented in every environment.

At the same time, TypeScript **does not change the meaning or behavior** of JavaScript code - it only adds a layer of type checking *before the program runs*.

---

### 2. Example: any JS is valid TS

```javascript
// Regular JavaScript
function sum(a, b) {
  return a + b;
}
```

This code is **perfectly valid** in TypeScript too.
But in TS you can add types:

```javascript
function sum(a: number, b: number): number {
  return a + b;
}
```

Now the compiler checks that `a` and `b` are numbers, and the return value is `number` as well.

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### 3. How it works "under the hood"

TypeScript files (`.ts`):

1. **Go through type checking** (nothing is executed, only analysis);
2. Are then **transpiled** (translated) into plain JavaScript code;
3. The resulting `.js` can run in a browser or Node.js, without needing TS at runtime.

---

### 4. Why this matters

Thanks to this architecture, TypeScript:

- is fully **compatible with the existing JavaScript ecosystem** (npm, Node.js, React, etc.);
- **does not require changing the runtime**, engine, or browser;
- adds a **layer of safety and predictability** while staying "transparent" for execution.

---

### Summary

> TypeScript is a **superset** of JavaScript,
> because it **includes all of JavaScript + adds typing and new features**,
> while remaining **compatible with and compilable** to plain JavaScript.

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