Reserved Globals

-compat=strict|js — whether a program may declare its own binding named the same as an ambient global (Math/JSON/console/process/fetch/parseInt/... — "Tier 1"). Real browsers/Node genuinely allow `const Math = {}`; this compiler's default is deliberately stricter, closing the same class of silent-miscompile bug fixed for fs/path/etc. (a same-named local variable silently routing to the built-in instead of the user's own value).

This file only exercises the *default* (`strict`) behavior — `make examples` always compiles with default flags, and a real shadowing declaration would fail to compile under strict mode by design, so it can't appear as live top-level code here. To see the permissive escape hatch actually run, save this next to this file as shadow_demo.ts:

function useFakeMath: number { let Math = { random: : number => 42 } return Math.random } console.log(useFakeMath)

then: `klainmain -compat=js shadow_demo.ts && ./shadow_demo` prints 42 (the user's own value, not a real random number) — versus the same file compiled with no flag at all (or explicit -compat=strict): `klainmain shadow_demo.ts` fails with:

klainmain: parse error: 2:3: 'Math' is a reserved built-in name — pass -compat=js to allow shadowing it (matches real JS/browser behavior)

Constructor-style built-ins (Map/Set/Date/RegExp/URL/EventEmitter/... — "Tier 2") stay reserved under *either* mode, always — decided directly, see's Context: the parser commits to the built-in meaning of `new Map` from the bare token text alone, before any scope information exists, so there's no notion of "shadow it" to opt into.

// Ordinary, unshadowed use of a reserved name is completely unaffected by
// -compat=strict being the default — only *declaring* a colliding binding
// is ever rejected.
console.log(Math.floor(3.7))          // 3
console.log(parseInt('42') + 1)       // 43
console.log(process.argv.length >= 1) // true

View source on GitHubexamples/globals/reserved_globals.ts

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