Need clarification of the target and lib compiler options

Question

I find I\'m confused by the target and lib options and how they interact with the features supported in the source code. I feel the docs need improving a little so am asking

Answer 1

(This started as a comment but it got too long.)

It's a bit confusing partly because there's some history behind it. I'm not qualified to answer this authoritatively but I've been following TypeScript since early development and this is my understanding:

• --target tells the compiler what library version to include while compiling (for example ES5 will give a compiler error if you use Promise, but ES6 will know all about Promise) and what version of JS is emitted by the compiler (for example ES5 will down-compile class syntax, but ES6 will leave it in).
• --lib was added later to give you better control over what library version to use while compiling without changing the emitted JS target. For example, a common problem was that you may include polyfills for ES6 library features, such as Promise, but you want to target ES5 browsers by down-compiling class syntax. Before --lib was around you either had to target ES6 to avoid compile errors about Promise, then down-compile again using Babel, or you could target ES5 and provide your own type definition for Promise so that the compiler doesn't give you an error. Now with --lib you can simply say your --target ES5 and --lib ES6, and the compiler will not complain about Promise but still down-compile to ES5.
• Neither option will cause TS to emit any library polyfills (Promise, etc), as you evidently found out; it's your responsibility to provide the correct runtime libraries. It only emits a few down-level language compatibility helpers, like __extends and __awaiter (the difference being that class or async is not just an API that can be polyfilled at runtime, it's a language feature with syntax implications). The --lib option is just your way of getting the right level of compile checking based on what you know you are going to have at runtime.
• As for why there's both ES6 and ES2015, that's just because ECMAScript changed the name and TS left the old name as a valid option for backwards compatibility. :)

You'll find a lot of this covered in these TS issues:

• #4168 - Normalize our lib files by compiler settings
• #6974 - Proposal: Modularize Library