foibles/dist/esm/mixin.js

Composition and mixins and TypeScript classes

// Symbol used to mark if a certain mixin has been applied
const application = Symbol('decorator:application');
/**
 * Check if the given prototype has the given mixin applied to itself.
 *
 * @param object
 * @param mixin
 */
function has(object, mixin) {
    while (object != null) {
        if (object.hasOwnProperty(application) && object[application] === mixin) {
            return true;
        }
        object = Object.getPrototypeOf(object);
    }
}
/**
 * Turn a function into a fully featured mixin. The mixin function should be
 * a function which takes a single argument which is the class to extend and
 * the function must return an extended class.
 *
 * ```javascript
 * const ExampleMixin = toMixin(base => class extends base {
 *   ...
 * });
 * ```
 *
 * @param func
 */
export function toMixin(func) {
    /*
     * Define a function that checks if the mixin has already been applied
     * to the prototype chain. This allows mixins to use other mixins as needed
     * without them being applied twice.
     */
    const mixinOnce = function (base) {
        if (has(base.prototype, func))
            return base;
        const result = func(base);
        result.prototype[application] = func;
        return result;
    };
    /*
     * Define a custom hasInstance symbol so that instanceof can be used if
     * the environment supports it.
     */
    if (Symbol.hasInstance) {
        if (func.hasOwnProperty(Symbol.hasInstance)) {
            /*
             * Mixin has its own hasInstance, let the outer function delegate
             * to it.
             */
            Object.defineProperty(mixinOnce, Symbol.hasInstance, {
                value: function mixinHasInstance(other) {
                    return func[Symbol.hasInstance](other);
                }
            });
        }
        else {
            /*
             * The mixin function does not have a custom hasInstance, use our
             * own.
             */
            Object.defineProperty(mixinOnce, Symbol.hasInstance, {
                value: function mixinHasInstance(other) {
                    return has(other, func);
                }
            });
        }
    }
    return mixinOnce;
}
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