Why require() gives me an undefined?

Why require.js gives me an undefined?

I try to use require.js, and create a load file use data-main = "load" in html, here is load.js:

requirejs(["script/app.js"]);

and in app.js, when I tried to load method, I received cannot read property of undefined, so I console.log(enter), and I received undefined

define(function(require) {
    var enter = require("./modules");
    console.log(enter); // shows 'undefined'
}); 

and this is modules.js:

define("post", ["require", "exports"], function (require, exports) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    exports.post = void 0;
    function post(message) {
        console.log("! Message: " + message);
    }
    exports.post = post;
});
define("main", ["require", "exports", "post"], function (require, exports, post_1) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    exports.entry = void 0;
    function entry() {
        (0, post_1.post)("Hello, World!");
    }
    exports.entry = entry;
});

but modules.js is exist, value of 'enter' shouldn't be undefined. What's happen?