Using block-scopeddeclare const anddeclare let in global context has a potentially unwanted effect - they can be accessed using unqualified access, but not usingglobalThis. When this behavior have been implemented in TypeScript (microsoft/TypeScript#30510) all declarations in standard lib files have been changed todeclare var, so I believe it's something that generally can be recommended for external declarations too.

// Invaliddeclareconstfoo:string;

// Invalidexport{}declare global{constfoo:string;}

// Validexport{}declareconstfoo:string;