Herb Syntax Tree Printer

Package: @herb-tools/printer

---

AST printer infrastructure for lossless HTML+ERB reconstruction and AST-to-source code conversion for the Herb Parser Syntax Tree.

Installation

npm

npm add @herb-tools/printer

pnpm

pnpm add @herb-tools/printer

yarn

yarn add @herb-tools/printer

bun

bun add @herb-tools/printer

Usage

IdentityPrinter (Provides lossless reconstruction of the original source)

For lossless reconstruction of the original source:

import { IdentityPrinter } from '@herb-tools/printer'
import { Herb } from '@herb-tools/node-wasm'

await Herb.load()

const parseResult = Herb.parse(
  '<div class="hello" >  Hello  </div>',
  { track_whitespace: true }
)

const printer = new IdentityPrinter()
const output = printer.print(parseResult.value)

// output === '<div class="hello" >  Hello  </div>' (exact preservation)

Custom Printers

Create custom printers by extending the base Printer class and override specific visitors for custom behavior:

import { Printer } from "@herb-tools/printer"
import { HTMLAttributeNode } from "@herb-tools/core"

class CustomPrinter extends Printer {
  protected write(content: string) {
    super.write(content.toUpperCase())
  }

  protected visitHTMLAttributeNode(node: HTMLAttributeNode) {
    // do nothing to strip attributes
  }
}

and then printing the result using print

import { Herb } from "@herb-tools/node-wasm"

await Herb.load()

const parseResult = Herb.parse(
  '<div class="hello">  Hello  </div>',
  { track_whitespace: true }
)

const printer = new CustomPrinter()
const output = printer.print(parseResult.value)

// output === '<div >  HELLO  </div>'

Print Options

The printer supports options to control how nodes are printed:

import { IdentityPrinter, DEFAULT_PRINT_OPTIONS } from "@herb-tools/printer"
import type { PrintOptions } from "@herb-tools/printer"

const printer = new IdentityPrinter()

// Will throw error if node has parse errors (default behavior)
const output1 = printer.print(nodeWithErrors)

// Will print the node despite parse errors
const output2 = printer.print(nodeWithErrors, { ignoreErrors: true })

When ignoreErrors is false (default), the printer will throw an error if you attempt to print a node that contains parse errors. Set ignoreErrors to true to print nodes with errors, which can be useful for debugging or partial AST reconstruction.

Important

The Printer expects the source to be parsed using the track_whitespace: true parser option for accurate source reconstruction.

CLI Usage

# Basic round-trip printing
herb-print input.html.erb > output.html.erb

# Verify parser accuracy
herb-print input.html.erb --verify

# Show parsing statistics
herb-print input.html.erb --stats