Babel Debug Macros And Feature Flags

This provides debug macros and feature flagging.

Setup

The plugin takes 4 types options: flags, svelte, debugTools, and
externalizeHelpers. The importSpecifier is used as a hint to this plugin as
to where macros are being imported and completely configurable by the host.

Like Babel you can supply your own helpers using the externalizeHelpers
options.

``js
{
plugins: [
['babel-plugin-debug-macros', {
// @optional
debugTools: {
isDebug: true,
source: 'debug-tools',
// @optional
assertPredicateIndex: 0
},

flags: [
{ source: '@ember/env-flags', flags: { DEBUG: true } },
{
name: 'ember-source',
source: '@ember/features',
flags: {
FEATURE_A: false,
FEATURE_B: true,
DEPRECATED_CONTROLLERS: "2.12.0"
}
}
],

// @optional
svelte: {
'ember-source': "2.15.0"
},

// @optional
externalizeHelpers: {
module: true,
// global: '__my_global_ns__'
}
}]
]
}
`

Flags and features are inlined into the consuming module so that something like UglifyJS will DCE them when they are unreachable.

Simple environment and feature flags

`javascript
import { DEBUG } from '@ember/env-flags';
import { FEATURE_A, FEATURE_B } from '@ember/features';

if (DEBUG) {
console.log('Hello from debug');
}

let woot;
if (FEATURE_A) {
woot = () => 'woot';
} else if (FEATURE_B) {
woot = () => 'toow';
}

woot();
`

Transforms to:

`javascript
if (true / DEBUG /) {
console.log('Hello from debug');
}

let woot;
if (false / FEATURE_A /) {
woot = () => 'woot';
} else if (true) {
woot = () => 'toow';
}

woot();
`

warn macro expansion

`javascript
import { warn } from 'debug-tools';

warn('this is a warning');
`

Expands into:

`javascript
(true && console.warn('this is a warning'));
`

assert macro expansion

The assert macro can expand in a more intelligent way with the correct
configuration. When babel-plugin-debug-macros is provided with the
assertPredicateIndex the predicate is injected in front of the assertion
in order to avoid costly assertion message generation when not needed.

`javascript
import { assert } from 'debug-tools';

assert((() => {
return 1 === 1;
})(), 'You bad!');
`

With the debugTools: { assertPredicateIndex: 0 } configuration the following expansion is done:

`js
(true && !((() => { return 1 === 1;})()) && console.assert(false, 'this is a warning'));
`

When assertPredicateIndex is not specified, the following expansion is done:

`javascript
(true && console.assert((() => { return 1 === 1;})(), 'this is a warning'));
`

deprecate macro expansion

`javascript
import { deprecate } from 'debug-tools';

let foo = 2;

deprecate('This is deprecated.', foo % 2);
`

Expands into:

`javascript
let foo = 2;

(true && !(foo % 2) && console.warn('This is deprecated.'));
`

Externalized Helpers

When you externalize helpers you must provide runtime implementations for the
above macros. An expansion will still occur, however we will emit references to
those runtime helpers.

A global expansion looks like the following:

`javascript
import { warn } from 'debug-tools';

warn('this is a warning');
`

Expands into:

`javascript
(true && Ember.warn('this is a warning'));
`

While externalizing the helpers to a module looks like the following:

`javascript
import { warn } from 'debug-tools';

warn('this is a warning');
`

Expands into:

`javascript
(true && warn('this is a warning'));
`

Svelte

Svelte allows for consumers to opt into stripping deprecated code from your
dependecies. By adding a package name and minimum version that contains no
deprecations, that code will be compiled away.

For example, consider you are on ember-source@2.10.0 and you have no
deprecations. All deprecated code in ember-source that is <=2.10.0 will be
removed.

`

svelte: {
"ember-source": "2.10.0"
}

`

Now if you bump to ember-source@2.11.0 you may encounter new deprecations.
The workflow would then be to clear out all deprecations and then bump the
version in the svelte options.

`
svelte: {
"ember-source": "2.11.0"
}
``