typescript triple slash

Triple-slash directives are single-line comments containing a single XML tag. The contents of the comment are used as compiler directives.
Triple-slash directives are only valid at the top of their containing file.
Triple-slash references instruct the compiler to include additional files in the compilation process.
/// <reference path="..." />

For example, adding a lib="es2017.string" directive to one of the files in a compilation is equivalent to compiling with --lib es2017.string.
/// <reference lib="es2017.string" />
"foo".padStart(4);