Feature Request: Ability to opt out of liveBinding with exports of const values

[kwasimensah]

Suggestion

🔍 Search Terms

liveBinding Object defineProperty allocation export live view

✅ Viability Checklist

My suggestion meets these guidelines:

• [ x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [ x] This wouldn't change the runtime behavior of existing JavaScript code
• [ x] This could be implemented without emitting different JS based on the types of the expressions
• [ x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [x ] This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

opt out of liveBinding added in #35967 for const values

📃 Motivating Example

Doing some profiling of an app I'm creating and seeing a lot of calls to the get function of Object defineProperty that was added as part of #35967. These might be optimized away when not using --trace-allocation-stack-interval but it would be great to not have to rely on the optimizer to do the right thing here.

I originally wanted to add a flag that would make sure that

TypeScript/src/compiler/transformers/module/module.ts

Line 1147 in 1e65b33

createExportExpression(factory.getExportName(specifier), exportedValue, /* location */ undefined, /* liveBinding */ true)

doesn't unconditionally pass true here, but that would be a breaking change to existing Typescript code if the flag was used with a modifiable export (does the breaking change guideline count if someone needs to explicitly flag themselves into a behavior?)

If the exported value is const then you don't need a "live view" of it. But it's not clear if there is a good way to communicate const-ness at the time the export expression is created.

💻 Use Cases

I think I can work around this by saving the value of the re-export to a variable local to the the current script/module and using that. But that seems cumbersome.

[kwasimensah]

Looking into this some more, I think I actually want to edit

TypeScript/src/compiler/transformers/module/module.ts

Line 1510 in 1e65b33

statements = appendExportsOfDeclaration(statements, importBinding, /* liveBinding */ true);

A really simple example of the issue is

import {createHash} from "crypto";

export {createHash}

Output

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.createHash = void 0;
const crypto_1 = require("crypto");
Object.defineProperty(exports, "createHash", { enumerable: true, get: function () { return crypto_1.createHash; } });

Compiler Options

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "target": "ES2017",
    "jsx": "react",
    "module": "ESNext",
    "moduleResolution": "node"
  }
}

Playground Link: Provided