add default export

[huan]

Enable us to use import levelup from 'levelup' by adding default export.

[ralphtheninja]

LGTM

[juliangruber]

Should we document this as well? You too often have to guess how to import a node module

[ralphtheninja]

@zixia Mind adding something to the README?

[huan]

@ralphtheninja Ok, I will try to add some docs for this.

[huan]

I just add some docs about to introduce the require/import and typing.

Do not know where is the best position to add them, so I put them after the Promise part.

[juliangruber]

I feel this could be simpler, what about having this in the first code example:

var levelup = require('levelup')
var leveldown = require('leveldown')

// or
import levelup from 'levelup'
import leveldown from 'leveldown'

[huan]

@juliangruber No problem for me. I can modify the PR if needed.

Actually, this PR does not change any import/require usage than before.

It only affects the rollup tools behavior. (see my reference above, and this one: rollup/rollup#1267 (comment))

[juliangruber]

Well, it adds the ability to import the default export for any es6 modules compliant environment.

[ralphtheninja]

By using require

[ralphtheninja]

By using ES6 import

[ralphtheninja]

Maybe remove this section? Not sure what it has to do with this PR? (And we did remove typings for now until a later version)

[huan]

Edited by following reviews from @ralphtheninja

[MeirionHughes]

I think typescript will have a hard time supporting both with this because you're mixing module kinds:

export = levelup;
export default levelup;

error:

[ts] An export assignment cannot be used in a module with other exported elements.

So to type this I'd have to pick one kind for ts users. Currently, ts users have:

import * as levelup from 'levelup';

switching over to the default export would mean ts-users would need to do:

import levelup from 'levelup';

This does work for me locally and moving forward, using the es6 standard appears to make sense.

I would caution though that the way es6 module loaders deal with es6 modules is not the same as commonjs: which is why nodejs still isn't switching over: https://medium.com/the-node-js-collection/an-update-on-es6-modules-in-node-js-42c958b890c

[huan]

@MeirionHughes Yes, you can not use export = with export default together.

We should either to use

1. in ES6/TypeScript

export { levelup }
export default levelup

or

2. in traditional

export = levelup.default = levelup.levelup = levelup

I believe in both of above two kinds, the user could always use import * as levelup from 'levelup' in their code.

[juliangruber]

It has to stay

const levelup = require('levelup')

for "older" systems though, is there a way to keep this and make es6 environments happy?

[ralphtheninja]

I'm good with the changes and agree with @juliangruber, whatever you want to pick for es6 stuff should not mess up current functionality.

[huan]

@juliangruber Yes, I agree with you that we have to stay const levelup = require('levelup').

This PR did not affect the "older" systems usage because it only adds a default export to the library.

[huan]

When will we publish the latest version to NPM? Thanks!

[MeirionHughes]

I've got the typings nearly ready to add back in to; waiting on leveldown.