What does /// <reference types="node" /> mean?

Question

I was having a problem with typescript and node and stumbled on a fix (I was getting a TS complaint that 'process' was not defined, as in process.env in Node). All I had to do was paste one line at the top of my file:

/// <reference types="node" /> 

I don't understand the syntax. Is the typescript compiler somehow reading a comment? How does this work?

Answer 1

These are referred to as "Triple Slash Directives" (Typescript docs)

As stated on the first line of that link:

Triple-slash directives are single-line comments containing a single XML tag. The contents of the comment are used as compiler directives.

So yes, the typescript compiler is picking this up during compilation and taking the appropriate action.

In this case, since you are using a types directive, you are telling the compiler that this file has a dependency on the node typings.

That said, the docs also state that for types directives:

Use these directives only when you're authoring a d.ts file by hand

So if you have added this do a .ts file rather than a .d.ts file, you may be setting yourself up for further problems.

For declaring a dependency on an @types package in a .ts file, use --types on the command line or in your tsconfig.json instead. See using @types, typeRoots and types in tsconfig.json files for more details.