# @dojo/has

[![Build Status](https://travis-ci.org/dojo/has.svg?branch=master)](https://travis-ci.org/dojo/has)
[![codecov](https://codecov.io/gh/dojo/has/branch/master/graph/badge.svg)](https://codecov.io/gh/dojo/has)
[![npm version](https://badge.fury.io/js/%40dojo%2Fhas.svg)](https://badge.fury.io/js/%40dojo%2Fhas)

A feature detection library.

This package provides an API for expressing feature detection to allow developers to branch code based upon the
detected features.  The features can also be asserted statically, thereby allowing integration into a build
optimization tool that can be used to make certain branches of code "dead" which can be elided during a build
step.  The `has` module is also capable of allowing conditional loading of modules with certain loaders.

**WARNING** This is *beta* software.  While we do not anticipate significant changes to the API at this stage, we may feel the need to do so.  This is not yet production ready, so you should use at your own risk.

- [Usage](#usage)
- [Features](#features)
    - [Feature Branching](#feature-branching)
	- [Included Features](#included-features)
	- [Adding a Feature Test/Feature Detection](#adding-a-feature-testfeature-detection)
	- [Conditional Module Loading](#conditional-module-loading)
	- [Static Features](#static-features)

## Usage

To use `@dojo/has`, simply install the package:

```bash
npm install @dojo/has
```

and import it into your application,

```typescript
import has from '@dojo/has';

if (has('some-feature')) {
	/* use some feature */
}
```

## Features

### Feature Branching

The most common feature is branching in code based upon a feature flag.  `has()` essentially manages feature
flags, returning either a *truthy* value if the feature is present or a *falsey* value if the feature isn't present.
The `has()` module is the default export of the main module of the package:

For example:

```typescript
import has from '@dojo/has';

if (has('host-browser')) {
	/* Browser Related Code */
}
else {
	/* Non-Browser Related Code */
}
```

`has()` can be used in any conditional expression, like a ternary operator:

```typescript
function getArrayOrString(): number[] | string {
	return has('some-feature') ? [ 1, 2, 3 ] : '[ 1, 2, 3 ]';
}
```

### Included Features

Other Dojo 2 packages leverage the `has()` package to express features that are then used within the package.  Because
of this, there are very few features expressed in this foundational package.  The intention is that this package is
used to enable a developer to express other features.  The flags though that are included in this package are:

|Feature Flag|Description|
|------------|-----------|
|`debug`|Provides a way to code path for code that is only usable when debugging or providing enhanced diagnostics that are not desired in a *production* build.  Defaults to `true` but should be configured statically as `false` in production builds.|
|`host-browser`|Determines if the current environment contains a `window` and `document` object in the global context, therefore it being generally safe to assume the code is running in a browser environment.|
|`host-node`|Attempts to detect if the environment appears to be a node environment.|

### Adding a Feature Test/Feature Detection

The main module of the package exports a function named `add()` which allows the addition of features flags.  The feature
tests can be expressed as a static value, a function which will be lazily evaluated when the feature flag is first
requested from `has()`, or a thenable (an object with a `then` method, like a Promise).  Once evaluated, the value is cached.

An example of adding a feature:

```typescript
import { add } from '@dojo/has';

add('my-feature', true);
add('my-lazy-feature', () => {
	/* will not be called yet */
	return true;
});
add('my-promise', new Promise((resolve) => {
	// start some asynchronous task
	resolve(true);
}));

if (has('my-lazy-feature')) { /* feature function called here */
	/* do something */
}
```

If a feature flag is already added, the value can be overridden by supplying `true` as the 3rd argument of the `add()`
function:

```typescript
add('my-feature', false, true);
```

The module also has an `exists()` function which returns `true` if the feature flag has been added to `has()`:

```typescript
import { exists, add } from '@dojo/has';

if (!exists('my-feature')) {
	add('my-feature', false);
}
```

Note that if a thenable is passed to `add`, `exists` and `has` will return `false` until the thenable is resolved and a
proper value can be returned.

### Conditional Module Loading

When using an AMD loader that supports loader plugins (such as [`@dojo/loader`](https://github.com/dojo/loader)) then
`@dojo/has` can be used to conditionally load modules or substitute one module for another.  The module ID is specified
using the plugin syntax followed by the feature flag and a ternary operator of what to do if the feature is *truthy*
or *falsey*:

```typescript
import foo from '@dojo/has!host-browser?foo/browser:foo/node';

/* foo is now the default export from either `foo/browser` or `foo/node` */
```

The module IDs supplied in the ternary operator can be specified as absolute MIDs or relative MIDs based on the loading
module, just as if you were just directly importing the module.

When using TypeScript, TypeScript will not be able to automatically resolve the module shape, therefore you will often
have to make a global declaration of the module that is in the scope of the project where the module name matches the
full MID you will be importing:

```typescript
declare module '@dojo/has!host-browser?foo/browser:foo/node' {
	export * from 'foo/browser'; /* Assumes that foo/browser and foo/node have the same shape */
}
```

### Static Features

Features can also be defined statically, before the module is loaded, in the global scope.  The main use case is when
it is not desirable to detect these features from the environment (because they may not be accurate, like when using
a build tool).  The features can only be specified before the module is loaded for the first time and cannot be
changed once the module is loaded.  The values specified in the static features will *always* be returned from `has()`
irrespective of how those features a subsequently defined using `add()`, even if `override` is specified.  In addition,
if a value is being added via `add()` that is already defined as a static feature, it will still complete and not throw
although if specified as function, the function will never be invoked.

To specify the features, the global variable `DojoHasEnvironment` needs to be specified with a property of `staticFeatures`
which is a simple map of the features:

```typescript
window.DojoHasEnvironment = {
	staticFeatures: {
		'host-node': true,
		'host-browser': false,
		'my-feature': 2
	}
};
```

`staticFeatures` can also be specified as a function, which returns an map of the features:

```typescript
window.DojoHasEnvironment = {
	staticFeatures: function () {
		return { 'host-node': true, 'host-browser': false, 'my-feature': 2 };
	}
};
```

This function will be run once when the module is loaded and the values returned from the function will be used as the
static features.

## How do I contribute?

We appreciate your interest!  Please see the [Dojo 2 Meta Repository](https://github.com/dojo/meta#readme) for the
Contributing Guidelines.

### Code Style

This repository uses [`prettier`](https://prettier.io/) for code styling rules and formatting. A pre-commit hook is installed automatically and configured to run `prettier` against all staged files as per the configuration in the projects `package.json`.

An additional npm script to run `prettier` (with write set to `true`) against all `src` and `test` project files is available by running:

```bash
npm run prettier
```

## Installation

To start working with this package, clone the repository and run `npm install`.

In order to build the project run `grunt dev` or `grunt dist`.

## Testing

Test cases MUST be written using [Intern](https://theintern.github.io) using the Object test interface and Assert assertion interface.

90% branch coverage MUST be provided for all code submitted to this repository, as reported by istanbul’s combined coverage results for all supported platforms.

To test locally in node run:

`grunt test`

To test against browsers with a local selenium server run:

`grunt test:local`

To test against BrowserStack or Sauce Labs run:

`grunt test:browserstack`

or

`grunt test:saucelabs`

## Licensing information

The original Dojo 1 `has()` API was based upon Peter Higgin's [has.js](https://github.com/phiggins42/has.js/).

© [JS Foundation](https://js.foundation/) & contributors. [New BSD](http://opensource.org/licenses/BSD-3-Clause) license.