Note: This feature is available in Web Workers.
The read-only actualBoundingBoxDescent property of the TextMetrics interface is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the bottom of the bounding rectangle used to render the text, in CSS pixels.
const canvas = document.createElement("canvas");
const ctx = canvas.getContext("2d");
const text = ctx.measureText("foo"); // returns TextMetrics object
text.actualBoundingBoxDescent; // 0;