Optimize ES2015 enums

[filipesilva]

🚀 Feature request

Description

Using the ES2015 target, TypeScript transpiles enums as consts plus property assignments.

As an example, the ChangeDetectionStrategy enum in https://github.com/angular/angular/blob/13eb57a59fd4db27e88edb188947297b6637177f/packages/compiler/src/core.ts#L82-L85:

export enum ChangeDetectionStrategy {
  OnPush = 0,
  Default = 1
}

Is transpiled to ES2015 as:

const ChangeDetectionStrategy = {
    OnPush: 0,
    Default: 1
};
ChangeDetectionStrategy[ChangeDetectionStrategy.OnPush] = "OnPush";
ChangeDetectionStrategy[ChangeDetectionStrategy.Default] = "Default";

The property assignment in the transpiled code prevents the class from being removed by minifiers such as terser.

Describe the solution you'd like

Build Optimizer converts ES2015 enums in such a way that they are not retained after minification.

[michaeljota]

You could try const enum. They are not available at runtime, and can only be used by the reference.

[filipesilva]

I've seen this happen on the Angular core esm2015 bundle, but the fesm2015 uses the transform we currently have. Might be a matter of detection.

@michaeljota we want to be able to do these transformations on arbitrary third party code, where a user most likely not has the option to change underlying source.

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