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 eval¹:

console.log(eval("5 + 2")) // 7

const obj = {
  hello() {
    console.log("Hello!")
  },
}

// Meta level
for (const key of Object.keys(obj)) {
  console.log(key)
}

All Object.* methods can be considered metaprogramming functionality

Kinds of metaprogramming

Reflective metaprogramming means that a program processes itself. Kiczales et al². [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

Examples

Introspection

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.)

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 were created to fill that gap.

Footnotes