enums are not compiled as numbers when declaration flag is false in tsconfig.json

[akosbordas]

TypeScript Version: 2.02

Expected behavior:

I have a const enum in typescript:

const enum LogLevel {
    TRACE = 0,
    DEBUG = 1,
    INFO = 2,
    WARN = 3,
    ERROR = 4,
    SILENT = 5
}

Based on the typescript spec the following field:

private foo: number = LogLevel.DEBUG;

should be compiled as:

this.foo = 1 /* DEBUG */;

Actual behavior:

When I use tsc from the command line (Windows) it works as expected. But when it is compiled with awesome-typescript-loader in a webpack project (which uses the typescript.js from node_modules as opposed to the tsc.js which is used by tsc), then the enum constant is not getting inlined:

this.foo = LogLevel.DEBUG;

It turned out it was caused by the declaration option in tsconfig.json. If it is set to false, the two compilations produce the above inconsistent result. But when it is set to true, it works as expected. Not sure why this flag has such an effect on the outcome.

I created a github project where the problem is reproducible.

https://github.com/akosbordas/typescript-enum-issue

After you changed the declaration flag you just have to run ng build and check the main.bundle.js in dist folder.

[RyanCavanaugh]

This is an issue with how awesome-typescript-loader is using the compiler API.

awesome-typescript-loader uses the TypeScript transpileModule function in "fast mode"; "fast mode" is enabled when the declaration flag is not set. transpileModule does a pure syntactic transform of each file; in this mode, const enums are not resolved (because its a per-file transform) and their values are not inlined.

transpileModule should not be used unless an error-free compilation of the same sources has occurred under the --isolatedModules flag, which verifies that it is safe to use transpileModule on a particular compilation.

I would log a bug with awesome-typescript-loader -- their behavior here is not sound. Hopefully there's some way to configure it to not use transpileModule?

[akosbordas]

Thank you for your answer! I created an issue for awesome-typescript-loader as well. #245