TypeScriptのデコレータでクラス宣言順でハマる例

index.js

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __metadata = (this && this.__metadata) || function (k, v) {
    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
};
function methodDecorator(target, key, descriptor) { return target; }
class Hoge {
    get(asdf) {
    }
}
__decorate([
    methodDecorator,
    __metadata("design:type", Function),
    __metadata("design:paramtypes", [Asdf]),
    __metadata("design:returntype", void 0)
], Hoge.prototype, "get", null);
class Asdf {
}
//# sourceMappingURL=index.js.map

index.ts

function methodDecorator(target, key, descriptor) { return target}

class Hoge {
    a: Asdf;

    @methodDecorator
    get(asdf: Asdf) {
    }
}

class Asdf {
}

package.json

{
  "devDependencies": {
    "typescript": "^2.6.2"
  }
}

tsconfig.json

{
    "compilerOptions": {
        "sourceMap": true,
        "target": "es2017",
        "emitDecoratorMetadata": true,
        "experimentalDecorators": true,
        "declaration": false,
        "noImplicitAny": false,
        "noImplicitReturns": false,
        "strictNullChecks": false,
        "baseUrl": ".",
        "skipLibCheck": true,
        "lib": [
            "es5",
            "es6",
            "es2015",
            "dom"
        ]
    },
    "exclude": [
        "node_modules"
    ]
}

yarn.lock

# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
# yarn lockfile v1


typescript@^2.6.2:
  version "2.6.2"
  resolved "https://registry.yarnpkg.com/typescript/-/typescript-2.6.2.tgz#3c5b6fd7f6de0914269027f03c0946758f7673a4"

ES5がターゲットだと下記のようになり、この現象は発生しなくなります。

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __metadata = (this && this.__metadata) || function (k, v) {
    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
};
function methodDecorator(target, key, descriptor) { return target; }
var Hoge = /** @class */ (function () {
    function Hoge() {
    }
    Hoge.prototype.get = function (asdf) {
    };
    __decorate([
        methodDecorator,
        __metadata("design:type", Function),
        __metadata("design:paramtypes", [Asdf]),
        __metadata("design:returntype", void 0)
    ], Hoge.prototype, "get", null);
    return Hoge;
}());
var Asdf = /** @class */ (function () {
    function Asdf() {
    }
    return Asdf;
}());
//# sourceMappingURL=index.js.map