The @dojo/has repository has been deprecated and merged into @dojo/framework

You can read more about this change on our blog. We will continue providing patches for has and other Dojo 2 repositories, and a CLI migration tool is available to aid in migrating projects from v2 to v3.

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@dojo/has

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 create "dead" code branches which can be elided during a build step. The has module is also capable of allowing conditional loading of modules with certain loaders.

• Usage
• Features
  • Feature Branching
  • Included Features
  • Adding a Feature Test/Feature Detection
  • Conditional Module Loading
  • Static Features

Usage

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

npm install @dojo/has

and import it into your application,

import has from '@dojo/has';

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

Features

Feature Branching

The most common use case 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:

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:

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 create a 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 is 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:

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:

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

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

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) 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:

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 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:

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, for example 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 are subsequently defined using add(), even if an 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. If specified as a 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:

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

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

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.

Code Style

This repository uses prettier 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 project's 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:

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 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.

© 2018 JS Foundation & contributors. New BSD license.