Figma Function to generate font-metrics for use in libraries like SeekOSS Capsize

figma-font-metrics.ts

type FontMetrics = {
	capHeight: number,
	ascent: number,
	descent: number,
	lineGap: number,
	unitsPerEm: number
}


async function makeMetrics(fontName: FontName, unitsPerEm: number){
await figma.loadFontAsync(fontName);



let fontMetrics: FontMetrics = {
  capHeight: 0,
  ascent: 0,
	descent: 0,
  lineGap: 0,
  unitsPerEm: unitsPerEm
};


let frame = figma.createFrame()
let text = figma.createText()

text.fontName = fontName
text.characters = "Hg"
frame.appendChild(text)
text.fontSize = fontMetrics.unitsPerEm
text.lineHeight = {unit: "AUTO"}
text.x = text.y =  frame.x = frame.y = 0
frame.resizeWithoutConstraints(text.width,text.height)


//Get cap height
let clone = text.clone()
clone.characters = "H"
let flat = figma.flatten([clone])
frame.appendChild(flat)
fontMetrics.capHeight = Number(flat.height.toFixed(2))

//get ascent + descent
fontMetrics.ascent = Number((fontMetrics.capHeight + flat.y).toFixed(2))
fontMetrics.descent = Number((frame.height - fontMetrics.ascent).toFixed(2))

frame.remove()

return fontMetrics
}


async function getMetrics(fontName: FontName){
let fontMetrics = await makeMetrics(fontName, 1000)
if(!!Object.values(fontMetrics).find((value: number) => !!(value % 1))){
	fontMetrics = await makeMetrics(fontName,2048)
}
return fontMetrics
}

print(await getMetrics({family: "Roboto", style: "Bold"}))