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