What are typescript `--lib` library files?
typescript, typescript2.2
Solution
They tell the typescript compiler that those type libraries are available during the run-time and it won't complain that your target es version is missing features.
Your config is targeting `es5`, but you're telling typescript that "es2015" features will be available - so for example Promise and Map. Similarly you have "dom", so typescript knows that you have `window` and browser-dom features.
As far as I'm aware, it won't alter the output... typescript will simply error when you try to use features that are not present in your target es version.
You'd do this when you want to target old browsers, but also want to use, for example, Promises. So we tell the compiler its available with `lib:["es2015.promise"]` then you run something like `core-js` or `bluebird` at execution-time to polyfill the Promise functionality.
Problem
There is a similar question. That covers what typescript does with the option. The below question and answer covers where the `lib` files come from at runtime. When I looked up the description for the `--lib` compiler option it says: List of library files to be included in the compilation. What are these library files and how are they included? The repository I'm studying has the following settings? ``` "compilerOptions": { "target": "es5", "module": "es2015", "lib": ["es2015", "dom"] }, ``` How would the output be different if `--lib` were not set to `["es2015", "dom"]`?