The read-only fontBoundingBoxAscent property of the TextMetrics interface returns the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute, to the top of the highest bounding rectangle of all the fonts used to render the text, in CSS pixels.

A number, in CSS pixels.

The code below shows how you can get the fontBoundingBoxAscent for some text in a particular font.

const canvas = document.createElement("canvas");
const ctx = canvas.getContext("2d");

ctx.font = "25px serif";
const text = "Foo";

const textMetrics = ctx.measureText("foo"); // returns TextMetrics object
const ascentCssPixels = textMetrics.fontBoundingBoxAscent;

The ascent in CSS pixels for the text "Foo" in a 25px serif font is shown below.

• TextMetrics.fontBoundingBoxDescent
• TextMetrics