Imported from terminalskills/skills (skills/opentype-js/SKILL.md). Install upstream with npx skills add terminalskills/skills --skill opentype-js. Copyright stays with the author (Apache-2.0).

opentype.js

Read and write OpenType fonts. Access every glyph, measure text, convert to SVG paths.

Setup

# Install opentype.js for font parsing and manipulation.
npm install opentype.js

Loading a Font

// src/fonts/load.ts — Load a font from file or URL and read metadata.
import opentype from "opentype.js";
import fs from "fs";

// From file (Node.js)
const buffer = fs.readFileSync("./fonts/Inter-Regular.otf");
const font = opentype.parse(buffer.buffer);

console.log(font.names.fontFamily);       // "Inter"
console.log(font.names.designer);         // designer name
console.log(font.unitsPerEm);             // 2048
console.log(font.numGlyphs);              // total glyph count

Measuring Text

// src/fonts/measure.ts — Calculate text width and bounding box at a given size.
// Useful for layout engines and canvas text positioning.
import opentype from "opentype.js";

export function measureText(font: opentype.Font, text: string, fontSize: number) {
  const path = font.getPath(text, 0, 0, fontSize);
  const bb = path.getBoundingBox();
  const advance = font.getAdvanceWidth(text, fontSize);

  return {
    width: advance,
    height: bb.y2 - bb.y1,
    boundingBox: { x1: bb.x1, y1: bb.y1, x2: bb.x2, y2: bb.y2 },
  };
}

Rendering Text to SVG

// src/fonts/to-svg.ts — Convert a text string to an SVG path element.
// This produces resolution-independent text that doesn't require the font file.
import opentype from "opentype.js";

export function textToSvg(
  font: opentype.Font,
  text: string,
  fontSize: number,
  x: number,
  y: number
): string {
  const path = font.getPath(text, x, y, fontSize);
  const pathData = path.toPathData(2); // precision
  const bb = path.getBoundingBox();

  return `<svg xmlns="http://www.w3.org/2000/svg" viewBox="${bb.x1} ${bb.y1} ${bb.x2 - bb.x1} ${bb.y2 - bb.y1}">
  <path d="${pathData}" fill="currentColor"/>
</svg>`;
}

Accessing Individual Glyphs

// src/fonts/glyphs.ts — Extract glyph outlines and metadata for specific characters.
import opentype from "opentype.js";

export function getGlyphInfo(font: opentype.Font, char: string) {
  const glyph = font.charToGlyph(char);
  const path = glyph.getPath(0, 0, 72);

  return {
    name: glyph.name,
    unicode: glyph.unicode,
    advanceWidth: glyph.advanceWidth,
    pathData: path.toPathData(2),
    commands: path.commands,
  };
}

// List all glyphs in a font
export function listGlyphs(font: opentype.Font) {
  const glyphs: { index: number; name: string; unicode: number | undefined }[] = [];
  for (let i = 0; i < font.numGlyphs; i++) {
    const g = font.glyphs.get(i);
    glyphs.push({ index: i, name: g.name, unicode: g.unicode });
  }
  return glyphs;
}

Font Subsetting

// src/fonts/subset.ts — Create a subset font containing only the glyphs needed
// for a specific string. Reduces font file size for web embedding.
import opentype from "opentype.js";
import fs from "fs";

export function subsetFont(font: opentype.Font, chars: string, outputPath: string) {
  const glyphs = [font.glyphs.get(0)]; // always include .notdef
  const seen = new Set<number>();

  for (const char of chars) {
    const glyph = font.charToGlyph(char);
    if (glyph.index !== 0 && !seen.has(glyph.index)) {
      glyphs.push(glyph);
      seen.add(glyph.index);
    }
  }

  const subset = new opentype.Font({
    familyName: font.names.fontFamily?.en || "Subset",
    styleName: font.names.fontSubfamily?.en || "Regular",
    unitsPerEm: font.unitsPerEm,
    ascender: font.ascender,
    descender: font.descender,
    glyphs,
  });

  const buffer = Buffer.from(subset.download() as any);
  fs.writeFileSync(outputPath, buffer);
}