JavaScript

There were (and will continue to be) a lot of competitors in Some syntax to JavaScript compilers. TypeScript is different from them in that Your JavaScript is TypeScript. Here's a diagram:

JavaScript is TypeScript

However, it does mean that you need to learn JavaScript (the good news is you only need to learn JavaScript). TypeScript is just standardizing all the ways you provide good documentation on JavaScript.

• Just giving you a new syntax doesn't help catch bugs - but might help you write cleaner / less bugs (e.g. CoffeeScript).

• Creating a new language abstracts you too far from your runtimes and communities - but might help on-board you easier if its an already familiar flavour (e.g. Dart - closer for Java / C# devs).

TypeScript is just JavaScript with docs.

JSNext is open to interpretation - not everything proposed for the next version of JS actually makes it to browsers. TypeScript only adds support for proposals once they reach stage 3.

Making JavaScript Better

TypeScript will try to protect you from portions of JavaScript that never worked (so you don't need to remember this stuff):

[] + []; // JavaScript will give you "" (which makes little sense), TypeScript will error

//
// other things that are nonsensical in JavaScript
// - don't give a runtime error (making debugging hard)
// - but TypeScript will give a compile time error (making debugging unnecessary)
//
{} + []; // JS : 0, TS Error
[] + {}; // JS : "[object Object]", TS Error
{} + {}; // JS : NaN or [object Object][object Object] depending upon browser, TS Error
"hello" - 1; // JS : NaN, TS Error

function add(a,b) {
  return
    a + b; // JS : undefined, TS Error 'unreachable code detected'
}

Essentially TypeScript is linting JavaScript. Just doing a better job at it than other linters that don't have type information.

You still need to learn JavaScript

That said TypeScript is very pragmatic about the fact that you do write JavaScript so there are some things about JavaScript that you still need to know in order to not be caught off-guard. Let's discuss them next.

Note: TypeScript is a superset of JavaScript. Just with documentation that can actually be used by compilers / IDEs ;)