smartquote

Smart quote conversion utilities and ESLint plugin for typographically correct quotes.

Converts straight quotes (" and ') to their curly/smart equivalents (" " ' ').

Installation

npm install smartquote
# or
pnpm add smartquote

Quick Start

import { smartQuotes, SmartQuote } from 'smartquote';

const result = smartQuotes('She said "hello"');
// result === 'She said "hello"'

// Verify with constants
result.startsWith(`She said ${SmartQuote.LeftDouble}`); // true

API Reference

smartQuotes(text: string): string

Converts straight quotes to smart quotes using context-aware rules.

import { smartQuotes, SmartQuote } from 'smartquote';
const { LeftDouble, RightDouble, RightSingle } = SmartQuote;

// Basic conversion
smartQuotes('"hello"');
// === `${LeftDouble}hello${RightDouble}`

// Apostrophes are detected between letters
smartQuotes("It's wonderful");
// === `It${RightSingle}s wonderful`

// Nested quotes work correctly
smartQuotes(`He said "She told me 'yes'"`);
// Outer double quotes, inner single quotes

smartQuoteMarkdown(text: string): string

Markdown-aware conversion that preserves straight quotes inside code blocks.

import { smartQuoteMarkdown } from 'smartquote';

const markdown = `
"This quote converts," she said.

\`\`\`javascript
const x = "stays straight";
\`\`\`

Use \`"straight"\` in inline code.
`;

const result = smartQuoteMarkdown(markdown);
// Prose gets smart quotes; code blocks are unchanged

Preserved regions:

• Fenced code blocks (```)
• Inline code (`)
• Indented code blocks (4 spaces or tab)

SmartQuote

Constants using Unicode escapes (immune to LLM normalization):

import { SmartQuote } from 'smartquote';

SmartQuote.LeftDouble     // \u201C "
SmartQuote.RightDouble    // \u201D "
SmartQuote.LeftSingle     // \u2018 '
SmartQuote.RightSingle    // \u2019 '
SmartQuote.StraightDouble // \u0022 "
SmartQuote.StraightSingle // \u0027 '

Streaming API

For processing text streams (like AI chat responses) without re-processing already-converted content.

smartQuoteAsyncIterable(source, options?)

Wraps an AsyncIterable<string> to convert quotes on the fly. Use this for plain text streams.

import { smartQuoteAsyncIterable } from 'smartquote';

for await (const chunk of smartQuoteAsyncIterable(textStream)) {
  process.stdout.write(chunk);
}

// Disable markdown-aware conversion (converts quotes even in code blocks)
for await (const chunk of smartQuoteAsyncIterable(textStream, { disableMarkdown: true })) {
  process.stdout.write(chunk);
}

smartQuoteTransform(options?)

Returns a generic TransformStream for structured stream parts with a text-delta type.

import { smartQuoteTransform } from 'smartquote';

// Works with any stream of { type: 'text-delta', textDelta: string } parts
const stream = someTextStream.pipeThrough(smartQuoteTransform());

// Disable markdown-aware conversion
const stream = someTextStream.pipeThrough(smartQuoteTransform({ disableMarkdown: true }));

Streaming Options

Option	Type	Default	Description
disableMarkdown	boolean	false	When false, preserves straight quotes inside code blocks. Set to true to convert all quotes.

Why streaming buffers trailing quotes: When a chunk ends with ' (e.g., "don'"), we can't determine if it's an apostrophe or closing quote until the next chunk arrives. Both streaming APIs handle this automatically.

Vercel AI SDK Integration

For Vercel AI SDK v5+, use the dedicated smartquote/ai-sdk entry point which provides properly typed transforms:

npm install smartquote ai@>=5.0.0

// app/api/chat/route.ts
import { streamText } from 'ai';
import { anthropic } from '@ai-sdk/anthropic';
import { smartQuoteTransform } from 'smartquote/ai-sdk';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: anthropic('claude-sonnet-4-20250514'),
    messages,
    experimental_transform: smartQuoteTransform(),
  });

  return result.toDataStreamResponse();
}

The smartquote/ai-sdk module exports:

• smartQuoteTransform - Typed for StreamTextTransform<ToolSet>
• smartQuotes - Re-exported for convenience
• SmartQuote - Re-exported for convenience

ESLint Plugin

Enforce smart quotes in JSX/TSX at build time.

// eslint.config.js
import { plugin as smartQuotesPlugin } from 'smartquote/eslint';

export default [
  {
    files: ['**/*.tsx', '**/*.jsx'],
    plugins: {
      smartquote: smartQuotesPlugin,
    },
    rules: {
      'smartquote/smart-quotes': 'error',
    },
  },
];

What It Checks

• JSX text content: <p>Click "here"</p> (always user-facing)
• Allowlisted props: placeholder, title, alt, label, aria-label, aria-placeholder, aria-roledescription, aria-valuetext

Non-user-facing props (className, href, id, key, etc.) are ignored.

Options

// Add props to the default allowlist
'smartquote/smart-quotes': ['error', {
  additionalProps: ['data-tooltip', 'data-label']
}]

// Override defaults entirely
'smartquote/smart-quotes': ['error', {
  props: ['placeholder', 'title']
}]

Auto-fix

Run eslint --fix to automatically convert straight quotes to smart quotes in JSX.

Algorithm

Based on the algorithm from pensee.com/dunham/smartQuotes.html.

Opening quotes are used when:

1. At the beginning of text
2. After whitespace (space, tab, newline)
3. After left punctuation: ( [ { <
4. After an opening quote of the opposite type (nested quotes)

Closing quotes are used in all other cases.

Apostrophes are detected when a single quote appears between two letters (don't, it's).

Performance

The streaming API processes chunks incrementally (O(n) total) rather than re-processing accumulated text (O(n²)). For long AI responses, this can be 100x+ faster.

License

MIT