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Showing content from https://github.com/leebyron/ecmascript-more-export-from below:

leebyron/ecmascript-more-export-from: Proposal: add more export-from statements in ES7

This spec has been withdrawn in favor of the equivalent micro-proposals: export ns from and export default from.

Additional export-from statements in ES7

Stage: 1

Author: Lee Byron

Specification: Spec.md

AST: ESTree.md

Transpiler: See Babel's es7.exportExtensions option.

Problem statement and rationale

The export ___ from "module" statements are a very useful mechanism for building up "package" modules in a declarative way. In the ES6 spec, we can:

export through a single export with export {x} from "mod"
...optionally renaming it with export {x as v} from "mod"
We can also spread all exports with export * from "mod".

These three export-from statements are easy to understand if you understand the semantics of the similar looking import statements.

However, there are other import statements which would have very useful export-from forms.

Exporting the ModuleNameSpace object as a named export: export * as ns from "mod"
Forwarding the default export of the referenced module as a named export of this module: export v from "mod"

Import Statement Form [[ModuleRequest]] [[ImportName]] [[LocalName]] import v from "mod"; "mod" "default" "v" import * as ns from "mod"; "mod" "*" "ns" import {x} from "mod"; "mod" "x" "x" import {x as v} from "mod"; "mod" "x" "v" import "mod"; Export Statement Form [[ModuleRequest]] [[ImportName]] [[LocalName]] [[ExportName]] export var v; null null "v" "v" export default function f(){}; null null "f" "default" export default function(){}; null null "*default*" "default" export default 42; null null "*default*" "default" export {x}; null null "x" "x" export {x as v}; null null "x" "v" export {x} from "mod"; "mod" "x" null "x" export {x as v} from "mod"; "mod" "x" null "v" export * from "mod"; "mod" "*" null null Export Statement Form [[ModuleRequest]] [[ImportName]] [[LocalName]] [[ExportName]] export * as ns from "mod"; "mod" "*" null "ns" export v from "mod"; "mod" "default" null "v" Symmetry between import and export

There's a syntactic symmetry between the export-from statements and the import statements they resemble. There is also a semantic symmetry; where import creates a locally named binding, export-from creates an export entry.

As an example:

Is symmetric to:

However, where importing v introduces a name in the local scope, export-from v does not alter the local scope, instead creating an export entry.

The two proposed additions follow this same symmetric pattern:

Exporting a namespace exotic object without altering local scope:

// proposed:
export * as ns from "mod";
// symmetric to:
import * as ns from "mod";

Re-exporting a module's default export with a name:

// proposed:
export v from "mod";
// symmetric to:
import v from "mod";

Using the terminology of Table 42 in ECMA-262 v6 RC4, the export-from form can be created from the symmetric import form by setting export-from's [[ExportName]] to import's [[LocalName]] and export-from's [[LocalName]] to null.

Statement Form [[ModuleRequest]] [[ImportName]] [[LocalName]] [[ExportName]] import v from "mod"; "mod" "default" "v" export v from "mod"; "mod" "default" null "v" import * as ns from "mod"; "mod" "*" "ns" export * as ns from "mod"; "mod" "*" null "ns" import {x} from "mod"; "mod" "x" "x" export {x} from "mod"; "mod" "x" null "x" import {x as v} from "mod"; "mod" "x" "v" export {x as v} from "mod"; "mod" "x" null "v" import * from "mod" "mod" null null (many) export * from "mod"; "mod" "*" null null (many)

Note: import * from "mod" is only included for illustrative purposes. It is not in existing spec or part of this proposal.

Compound export-from statements

This proposal also includes the symmetric export-from for the compound imports:

// proposed
export v, {x, y as w} from "mod"
// symmetric to
import v, {x, y as w} from "mod"

As well as

// proposed
export v, * as ns from "mod"
// symmetric to
import v, * as ns from "mod"

One use case is to take the default export of an inner module and export it as the default export of the outer module. This is written as:

export default from "mod";

This is not additional syntax above what's already proposed. In fact, this is just the export v from "mod" syntax where the export name happens to be default. This nicely mirrors the other export default ____ forms in both syntax and semantics without requiring additional specification. An alteration to an existing lookahead restriction is necessary for supporting this case.

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