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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Decorators","articleBody":"\u003e ## ES7 proposal\n\u003e \n\u003e The ES7 proposal for decorators can be found here: https://github.com/wycats/javascript-decorators\n\u003e The ES7 proposal serves as the base of this proposal. Below are notes about how the type system \n## Decorator targets:\n### Class constructor\n\n``` ts\n@F(\"color\")\n@G\nclass Foo {\n}\n```\n\ndesugars to:\n\n``` js\nvar Foo = (function () {\n    function Foo() {\n    }\n    Foo = __decorate([F(\"color\"), G], Foo);\n    return Foo;\n})();\n```\n### Methods\n\n``` ts\nclass Foo {\n  @F(color)\n  @G\n  bar() { }\n}\n```\n\ndesugars to:\n\n``` js\nvar Foo = (function () {\n    function Foo() {\n    }\n    Foo.prototype.bar = function () {\n    };\n    Object.defineProperty(Foo.prototype, \"bar\", __decorate([F(color), G], Foo.prototype, \"bar\", Object.getOwnPropertyDescriptor(Foo.prototype, \"bar\")));\n    return Foo;\n})();\n```\n## Static method\n\n``` ts\nclass Foo {\n    @F(\"color\")\n    @G\n    static sMethod() {}\n}\n```\n\ndesugars to:\n\n``` js\nvar Foo = (function () {\n    function Foo() {\n    }\n    Foo.sMethod = function () {\n    };\n    Object.defineProperty(Foo, \"sMethod\", __decorate([F(\"color\"), G], Foo, \"sMethod\", Object.getOwnPropertyDescriptor(Foo, \"sMethod\")));\n    return Foo;\n})();\n```\n### Properties\n\n``` ts\nclass Foo {\n    @F(\"color\")\n    @G\n    prop: number;\n}\n```\n\ndesugars to:\n\n``` js\nvar Foo = (function () {\n    function Foo() {\n    }\n    __decorate([F(\"color\"), G], Foo.prototype, \"prop\");\n    return Foo;\n})();\n```\n### Method/Accessor formal parameter\n\n``` ts\nclass Foo {\n    method(@G a, @F(\"color\") b) {}\n}\n```\n\ndesugars to:\n\n``` js\nvar Foo = (function () {\n    function Foo() {\n    }\n    Foo.prototype.method = function (a, b) {\n    };\n    __decorate([G], Foo.prototype, \"method\", 0);\n    __decorate([F(\"color\")], Foo.prototype, \"method\", 1);\n    return Foo;\n})();\n```\n\nWhere the __decorate is defined as:\n\n``` js\nvar __decorate = this.__decorate || function (decorators, target, key, value) {\n    var kind = typeof (arguments.length == 2 ? value = target : value);\n    for (var i = decorators.length - 1; i \u003e= 0; --i) {\n        var decorator = decorators[i];\n        switch (kind) {\n            case \"function\": value = decorator(value) || value; break;\n            case \"number\": decorator(target, key, value); break;\n            case \"undefined\": decorator(target, key); break;\n            case \"object\": value = decorator(target, key, value) || value; break;\n        }\n    }\n    return value;\n};\n```\n## Decorator signatures:\n\nA valid decorator should be:\n1. Assignable to one of the Decorator types (ClassDecorator | PropertyDecorator | MethodDecorator | ParameterDecorator) as described below.\n2. Return a value (in the case of class decorators and method decorator) that is assignable to the decorated value.\n\n``` ts\ndeclare type ClassDecorator = \u003cTFunction extends Function\u003e(target: TFunction) =\u003e TFunction | void;\ndeclare type PropertyDecorator = (target: Object, propertyKey: string | symbol) =\u003e void;\ndeclare type MethodDecorator = \u003cT\u003e(target: Object, propertyKey: string | symbol, descriptor: TypedPropertyDescriptor\u003cT\u003e) =\u003e TypedPropertyDescriptor\u003cT\u003e | void;\ndeclare type ParameterDecorator = (target: Function, propertyKey: string | symbol, parameterIndex: number) =\u003e void;\n```\n### Notes:\n- Decorating a function declaration is not allowed as it will block hoisting the function to the top of the scope, which is a significant change in semantics. \n- Decorating function expressions and arrow functions are not supported. The same effect can be achived by applying the decorator function as `var x = dec(function () { });` \n- Decorating function formal parameters is currently not part of the ES7 proposal.\n- Decorators are not allowed when targeting ES3\n","author":{"url":"https://github.com/rbuckton","@type":"Person","name":"rbuckton"},"datePublished":"2015-03-07T03:14:53.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":139},"url":"https://github.com/2249/TypeScript/issues/2249"}