Pretext Text Measurement & Layout

Skill by ara.so — Daily 2026 Skills collection.

Pretext is a pure JavaScript/TypeScript library for fast, accurate, DOM-free multiline text measurement and layout. It avoids getBoundingClientRect and offsetHeight (which trigger expensive layout reflows) by implementing its own measurement logic using the browser's font engine as ground truth.

Installation

npm install @chenglou/pretext

Core Concepts

• prepare() / prepareWithSegments() — one-time analysis: normalize whitespace, segment text, measure via canvas. Cache and reuse this result.
• layout() / layoutWithLines() etc. — cheap hot path: pure arithmetic over cached widths. Call this on every resize, not prepare().
• Font string format — same as CanvasRenderingContext2D.font, e.g. '16px Inter', '700 18px "Helvetica Neue"'.

Use Case 1: Measure Paragraph Height (No DOM)