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content/20201022095438-javascript_metaprogramming.md

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+---
+id: b866160c-ac93-45af-8faf-1fd40f002f77
+title: JavaScript Metaprogramming
+---
+
+# Introduction
+
+In programming, there are levels:
+
+-   At the *base level* (also called: *application level*), code
+    processes user input.
+-   At the *meta level*, code processes base level code.
+
+## Examples
+
+An example of this is
+[eval](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval)[^1]:
+
+``` javascript
+console.log(eval("5 + 2")); // 7
+```
+
+``` javascript
+const obj = {
+  hello() {
+    console.log("Hello!");
+  },
+};
+
+// Meta level
+for (const key of Object.keys(obj)) {
+  console.log(key);
+}
+```
+
+All [Object.\*
+methods](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object#)
+can be considered metaprogramming functionality
+
+# Kinds of metaprogramming
+
+Reflective metaprogramming means that a program processes itself.
+Kiczales et al[^2]. \[2\] distinguish three kinds of reflective
+metaprogramming:
+
+-   **Introspection:** you have read-only access to the structure of a
+    program.
+-   **Self-modification:** you can change that structure
+-   **Intercession:** you can define the semantics of some language
+    operations
+
+## Introspection
+
+[Object.keys()](20200826201605-objects.org::*Object.keys) performs
+introspection
+
+## self-modification
+
+The following function `moveProperty` moves a property from a source to
+a target. It performs self-modification via the bracket operator for
+property access, the assignment operator and the delete operator. (In
+production code, you’d probably use property descriptors for this task.)
+
+``` javascript
+function moveProperty(source, propertyName, target) {
+  target[propertyName] = source[propertyName];
+  delete source[propertyName];
+}
+
+const obj1 = { prop: "abc" };
+const obj2 = {};
+moveProperty(obj1, "prop", obj2);
+
+console.log(obj1); // {}
+console.log(obj2); // { prop: 'abc' }
+```
+
+## intercession
+
+ECMAScript 5 doesn't support intercession.
+[Proxies](20201022094207-javascript_proxies) were created to fill that
+gap.
+
+# Footnotes
+
+[^1]: Don't ever use eval. If you think you're a clever clogs that can
+    work your away around the obvious security issues, then you
+    especially should not use eval.
+
+[^2]: "[The Art of the Metaobject
+    Protocol](http://mitpress.mit.edu/books/art-metaobject-protocol)" by
+    Gregor Kiczales, Jim des Rivieres and Daniel G. Bobrow. Book, 1991.