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Hoist initial assignment to exported names in cjs to they are not blocked by bindings made by __exportStar - #37093

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Wesley Wigham (weswigham) merged 4 commits into
microsoft:masterfrom
weswigham:exportstar-skip-locals
Feb 28, 2020
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Hoist initial assignment to exported names in cjs to they are not blocked by bindings made by __exportStar#37093
Wesley Wigham (weswigham) merged 4 commits into
microsoft:masterfrom
weswigham:exportstar-skip-locals

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@weswigham

@weswigham Wesley Wigham (weswigham) commented Feb 27, 2020 •
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We need this because now that __createBinding is used by __exportStar to create live-updating getters, if you had input like

export * from "mod";
export const nameFromMod = 0;

we'd output

__exportStar(exports, require("mod"));
exports.nameFromMod = 0;

the assignment will throw because the __exportStar made a getter which can't be overridden with a simple assignment. By emitting

exports.nameFromMod = void 0;
__exportStar(exports, require("mod"));
exports.nameFromMod = 0;

we prevent __exportStar from overwriting nameFromMod with a getter (so the assignment succeeds).

This also neatly ensures all export names are available to reexport when resolving within a circular module graph.

rbuckton
Ron Buckton (rbuckton) approved these changes Feb 28, 2020
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May need to discuss in design meeting.

@weswigham
Wesley Wigham (weswigham) merged commit c6c2c4c into microsoft:master Feb 28, 2020
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Wesley Wigham (weswigham) deleted the exportstar-skip-locals branch February 28, 2020 21:25
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Oscar Busk (oBusk) added a commit to oBusk/walkable-buffer that referenced this pull request May 13, 2020
@oBusk
chore: Don't use "export * from" to avoid required helper methods
7bdbaa5 
* https://github.com/microsoft/TypeScript/wiki/Breaking-Changes#exports-are-hoisted-and-initially-assigned
* microsoft/TypeScript#37093
@YowaiCoder

YowaiCoder commented May 19, 2020

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Missed the case of exported namespace (export * as foo from 'foo').

@ericjeney