@codepatch/javascript

Make small changes to your JavaScript code the easy way

Installation

npm install @codepatch/javascript

IMPORTANT: @codepatch/javascript is an ESM-only package. Read more.

Motivation

@codepatch/javascript is the ideal tool for programmatically making small & simple modifications to your JavaScript code. It works by parsing the code into an AST and then overriding parts of it. Learn more about the motivation behind Codepatch in the main repository.

As an introducing example, let's put a function wrapper around all array literals:

import { modify } from '@codepatch/javascript'

const code = `
const xs = [1, 2, [3, 4]]
const ys = [5, 6]
console.log([xs, ys])
`

const result = modify(code, (node, { source, override }) => {
  if (node.type === 'ArrayExpression') {
    override(`wrap(${source()})`)
  }
})

console.log(result.code)

Tip: To simplify following along the code above, you can have a look at the handled JavaScript code's AST.

Output:

const xs = wrap([1, 2, wrap([3, 4])])
const ys = wrap([5, 6])
console.log(wrap([xs, ys]))

Note that Codepatch is not a transpiler, so it's not ideal for large or complex changes, you'd want something like @babel/traverse for that.

Usage

How it Works

function modify(code, options = {}, manipulator)

Transform the code string with the function manipulator, returning an output object.

For every node in the AST, manipulator(node, helpers) is called. The recursive walk is an in-order, depth-first traversal, so children are handled before their parents. This makes it easier to write manipulators that perform nested transformations as transforming parents often requires transforming their children first anyway.

The modify() return value is an object with two properties:

• code – contains the transformed source code
• map – holds the resulting source map object, as generated by magic-string

Type casting a Codepatch result object to a string will return its source code.

Pro Tip: Don't know what a JavaScript AST looks like? Have a look at astexplorer.net and select the "JavaScript" language with the "acorn" parser to get an idea.

Options

All options are, as the name says, optional. If you want to provide an options object, its place is between the code string and the manipulator function.

Parser Options

Any options for the underlying acorn parser can be passed to options.parser:

modify(code, { parser: { sourceType: 'module' } }, (node, helpers) => {
  // Parse the `code` as an ES module
})

Custom Parser

You may pass a custom acorn parser as options.parser.customParser to use that instead of the default acorn version coming with this library:

import acorn from 'acorn'
import jsx from from 'acorn-jsx'

const options = {
  parser: {
    customParser: acorn.Parser.extend(jsx())
  }
}

modify(code, options, (node, helpers) => {
  // Parse the `source` as JSX
})

Source Maps

Codepatch uses magic-string under the hood to generate source maps for your code modifications. You can pass its source map options as options.sourceMap:

const options = {
  sourceMap: {
    hires: true
  }
}

modify(code, options, (node, helpers) => {
  // Create a high-resolution source map
})

Helpers

The helpers object passed to the manipulator function exposes three methods. All of these methods handle the current AST node (the one that has been passed to the manipulator as its first argument).

However, each of these methods takes an AST node as an optional first parameter if you want to access other nodes.

Example:

modify('x = 1', (node, { source }) => {
  if (node.type === 'AssignmentExpression') {
    // `node` refers to the `x = 1` Expression
    source() // returns "x = 1"
    source(node.right) // returns "1"
  }
})

source()

Return the source code for the given node, including any modifications made to child nodes:

modify('true', (node, { source, override }) => {
  if (node.type === 'Literal') {
    source() // returns "true"
    override('false')
    source() // returns "false"
  }
})

override(replacement)

Replace the source of the affected node with the replacement string:

const result = modify('4 + 2', (node, { source, override }) => {
  if (node.type === 'BinaryExpression') {
    override(source(node.left) + source(node.right))
  }
})

console.log(result.code)

Output:

42

parent(levels = 1)

From the starting node, climb up the syntax tree levels times. Getting an ancestor node of the program root yields undefined.

modify('x = [1]', (node, { parent }) => {
  if (node.type === 'Literal') {
    // `node` refers to the `1` literal
    parent() // same as parent(1), refers to the `[1]` expression
    parent(2) // refers to the `x = [1]` assignment expression
    parent(3) // refers to the `x = [1]` statement
    parent(4) // refers to the program as a whole (root node)
    parent(5) // yields `undefined`, same as parent(6), parent(7) etc.
  }
})

External Helper Access

If you want to extract manipulation behavior into standalone functions, you can import the helpers directly from the @codepatch/javascript package, where they are not bound to a specific node:

import { override } from '@codepatch/php'

// Standalone function, increments node's value if it's a number
const increment = node => {
  if (node.type === 'Literal' && typeof node.value === 'number') {
    override(node, String(node.value + 1))
  }
}

const result = modify('x = 1', node => {
  increment(node)
})

console.log(result.code)

Output:

x = 2

Asynchronous Manipulations

The manipulator function may return a Promise. If it does, Codepatch will wait for that to resolve, making the whole modify() function return a Promise resolving to the result object (instead of returning the result object directly):

const code = `
const content = curl("https://example.com")
`

const deferredResult = modify(code, async (node, { source, override }) => {
  if (node.type === 'CallExpression' && node.callee.name === 'curl') {
    // Replace all cUrl calls with their actual content

    // Get the URL (will only work for simple string literals)
    const url = node.arguments[0].value

    // Fetch the URL's contents
    const contents = await fetch(url).then(response => response.text())

    // Replace the cUrl() call with the fetched contents
    override(JSON.stringify(contents))
  }
})

// Result is not available immediately, we need to await it
deferredResult.then(result => {
  console.log(result.code)
})

Output:

const content = '<!doctype html>\n<html>\n[...]\n</html>'

Note: You have to return a Promise if you want to commit updates asynchronously. Once the manipulator function is done running, any override() calls originating from it will throw an error.

Related

@codepatch/javascript is part of the Codepatch family of tools. Codepatch is a collection of tools that make it easy to programmatically make simple modifications to code of various languages.

Check out the Codepatch repository to find tools for other languages or information about how to write your own Codepatch modifier.