CanvasRenderingContext2D.fillText(text, x, y [, maxWidth]);

CanvasRenderingContext2D.fillText(text, x, y [, maxWidth]);

[, maxWidth]的意思是，方括号代表可有可无，有fillText(text, x, y [, maxWidth])和fillText(text, x, y )2种形式
https://www.zhangxinxu.com/wordpress/2018/02/canvas-text-break-line-letter-spacing-vertical/