has.md

has

Detecting Features

The default export of dojo-core/has is a function which accepts a single parameter: the name of the feature to test for. If the feature is available, a truthy value is returned, otherwise a falsy value is returned:

import has from 'dojo-core/has';

if (has('dom-addeventlistener')) {
    element.addEventListener('click', function () { /* ... */ });
}

Adding Feature Detections

It's important to be able to add new feature tests that aren't provided out-of-the-box by dojo-core/has. This can be done easily by using the add function exported by the has module. It accepts two parameters: the name of the feature, and either an immediate value indicating its availability or a function that resolves to a value.

When a function is passed, the feature will be lazily evaluated - i.e. the function is not executed until the feature is actually requested. The return value is then cached for future calls for the same feature.

import { add as hasAdd } from 'dojo-core/has';
hasAdd('dom-queryselector', 'querySelector' in document && 'querySelectorAll' in document);

// Lazily executed; useful if a polyfill is loaded after page load
hasAdd('typedarray', function () {
    return 'ArrayBuffer' in window;
});

Accessing the Feature Cache

dojo-core/has maintains object hashes containing keys that correspond to all features that have been both registered and requested. The value associated with each feature name key corresponds to that feature's availability in the current environment. The object hash containing evaluated features is accessible via the cache export.