TypeScript uses lib: es2015 to compile itself although it is supposed to run in ES5 environments

[ajafff]

AFAIK TypeScript should be able to run in ES5 environments. If it's not, you can close this issue.

tsconfig-base.json contains "lib": ["es2015"] which allows uses of ES6 features within the compiler. This is basically the reason #28918 happened in the first place.

The change was made by @weswigham in #25140.

[j-oliveras]

Another ES2015 use (Array.prototype.fill): https://github.com/Microsoft/TypeScript/blob/bc59582c7dbfe8eed24fb00ad454fb3ab8fb14d2/src/compiler/core.ts#L2167-L2169

This is not solved by #28932.

[weswigham]

The change in #25140 was just to pull in the iterator types; so changing the lib to ["es5", "es2015.iterable"] should be OK and prevent us form using es6 methods unintentionally.

[j-oliveras]

I add this change to #28932.

EDIT: I can change it because of the fill usage (see my previous post). That is made by @Andy-MS on #26588.

[weswigham]

 export function fill<T>(length: number, cb: (index: number) => T): T[] { 
     return new Array(length).fill(0).map((_, i) => cb(i)); 
 } 

should probably just be

export function fill<T>(length: number, cb: (index: number) => T): T[] { 
    const result = new Array(length);
    for (const i = 0; i < length; i++) {
        result[i] = cb(i);
    }
    return result;
}

the fancy .fill and .map, while a neat shorthand, are not required.