Provide flag for projects to ban compile-time side-effects in root files
See original GitHub issueSearch Terms
- pure
- side-effects
- elision
- global-modifying
Suggestion
A new tsconfig
flag should be added that bans compile-time side-effects in project root files. This idea was originally proposed by @andrewbranch in response to an external pull request. The flag name can be discussed further, but examples might be noTypingSideEffects
, noGlobalModifyingModules
, noCompileTimeSideEffects
, pure
etc. The phrase “compile-time side-effects” refers to anything that can affect type-checking dependent source files outside of the standard export
syntax.
The TypeScript features that allow source files to modify typing information outside of their file scope include the following:
- ambient modules
- module augmentation
- global augmentation
- global variables
- global functions
- triple-slash directives
- … and potentially more
With this flag enabled, an error should appear on any usages of the above.
It may be worth allowing the flag (or a separate option) to specify allowlisted files that are expected to have side-effects. Notably, webpack’s sideEffects
option is a boolean | string[]
type for this purpose.
Comparison to bundler tree-shaking
This idea is inspired by bundler tree-shaking, but focuses on side-effects at compile-time rather than runtime. The ideas are similar but different enough to warrant a specific TypeScript flag.
For example, a file that calls console.log
on import has a runtime side-effect bundlers may care about but does not have any affect on typing. The allowlisted array of files with side-effects will likely be different between TypeScript and bundlers.
Use Cases
Banning compile-time side-effects allows future speed improvements. Notably, the compiler can now know whether source files and its import tree can be safely elided from a Program
if its exported identifiers are unused. This has potentially significant speed and memory improvements depending on the source file graph of a program.
A quick and unfinished implementation that only looks at export { ... } from "..."
syntax was created in PR #40966.
Examples
For a program with the following files, TypeScript should output:
a.ts(2,0): ‘someGlobalVar’ is a global variable, which is banned by the ‘noTypingSideEffects’ compiler option.
// tsconfig.json
{
"compilerOption": {
"noTypingSideEffects": true
},
"files": ["index.ts", "a.ts"]
}
// index.ts
import { a } from "./a";
console.log(someGlobalVar);
// a.ts
export const a = 1;
declare var someGlobalVar = 2;
Checklist
My suggestion meets these guidelines:
- This wouldn’t be a breaking change in existing TypeScript/JavaScript code
- This wouldn’t change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
- This feature would agree with the rest of TypeScript’s Design Goals.
Issue Analytics
- State:
- Created 3 years ago
- Comments:8 (8 by maintainers)
Top GitHub Comments
I recently spent a long time in this area at Google and wrote up my results here, in particular the discussion around noResove towards the end.
In my experience on a massive codebase, the vast majority of code does not rely on type-level side effects, but then also random small places all over do end up using it. For one broad example, the lit-element ecosystem (which declares HTML custom components) wants to use
declare global { interface HTMLTagNameMap { ... } }
to register their new elements as arguments todocument.createElement
.So I agree with the OP (in that changing the behavior here can have a big impact on build time), but also with the TS team that this is possibly too obscure to warrant a compiler flag.
I wonder if you could experiment with this at some level outside of TS. I am not so familiar with how multi-module builds work, but I could imagine something around gathering all the node_module entries that have opted into no type side effects and using that to control the list of inputs you pass to a TS compilation. That is more or less the solution we ended up with.
Examples of good feedback include: