ECMAScript Proposal: export default from

Stage: 1

Author: Lee Byron

Reviewers: Caridy Patiño

Specification: https://tc39.github.io/proposal-export-default-from/

AST: ESTree.md

Transpiler: See Babel's export-default-from plugin.

NOTE: Closely related to the export-ns-from proposal.

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 ECMAScript 2015 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 is an import statement which does not have corresponding export-from statement, exporting the ModuleNameSpace object as a named export.

Example:

export someIdentifier from "someModule";
export someIdentifier, { namedIdentifier } from "someModule";

Current ECMAScript 2015 Modules:

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 * as ns from "mod";	"mod"	"*"	null	"ns"
export {x} from "mod";	"mod"	"x"	null	"x"
export {x as v} from "mod";	"mod"	"x"	null	"v"
export * from "mod";	"mod"	"*"	null	null

Proposed addition:

Export Statement Form	[[ModuleRequest]]	[[ImportName]]	[[LocalName]]	[[ExportName]]
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 existing example:

import {v} from "mod";

If then v should be exported, this can be followed by an export. However, if v is unused in the local scope, then it has introduced a name to the local scope unnecessarily.

import {v} from "mod";
export {v};

A single "export from" line directly creates an export entry, and does not alter the local scope. It is symmetric to the similar "import from" statement.

export {v} from "mod";

This presents a developer experience where it is expected that replacing the word import with export will always provide this symmetric behavior.

However, when there is a gap in this symmetry, it can lead to confusing behavior:

"I would like to chime in with use-case evidence. When I began using ES6 (via babel) and discovered imports could be re-exported I assumed the feature set would be symmetrical only to find out quite surprisingly that it was not. I would love to see this spec round out what I believe is a slightly incomplete feature set in ES6."

• @jasonkuhrt

"I also bumped into this and even thought this was a bug in Babel."

• @gaearon

Proposed addition:

The proposed addition follows this same symmetric pattern:

Importing the "default" name (existing):

import v from "mod";

Exporting that name (existing):

import v from "mod";
export {v};

Symmetric "export from" (proposed):

export v from "mod";

Compound "export from" statements:

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

Importing the "default" name as well as named export (existing):

import v, { x, y as w } from "mod";

Exporting those names (existing):

import v, { x, y as w } from "mod";
export { v, x, w };

Symmetric "export from" (proposed):

export v, {x, y as w} from "mod";

Note: The compound form must also support export-ns-from should both proposals be accepted:

export v, * as ns from "mod";

Exporting a default as default:

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 can be written as:

export default from "mod";

This is symmmetric to the (existing) import form:

import default from "mod";
export { default };

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.

Common concerns:

Do we need this even through you can already do this with export { default } from "mod"?

Yes! It is true that you can already "export from" a module's default export without altering the local scope:

export { default } from "mod";

You can even rename the default:

export { default as someIdentifier } from "mod"

There is a benefit to this, it's explicit and it is symmetric with the "import" form:

import { default as someIdentifier } from "mod"

However the purpose of this proposal is not to enable a new use case, the purpose is to provide an expected syntactic form which currently does not exist, and which favors the "default" export.

That "import" example is not often typed, as there is a preferred shorter syntax:

import someIdentifier from "mod"

This proposal argues that a similar shorter syntax should also exist for a symmetric "export from" form:

export someIdentifier from "mod"

Table showing symmetry

Using the terminology of Table 40 and Table 42 in ECMAScript 2015, 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 {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 * as ns from "mod";	"mod"	"*"	"ns"
export * as ns from "mod";	"mod"	"*"	null	"ns"
export * from "mod";	"mod"	"*"	null	null (many)