Building a Canvas‑Based Rich Text Editor Similar to Word

The article introduces a canvas‑based approach to building a rich‑text editor, contrasting it with typical DOM‑based implementations that rely on the DOM . It starts by defining a JSON data model where each character is stored with style attributes such as value , color , size , bold , italic , etc.

A CanvasEditor class is created, initializing container, data, and options like page dimensions, padding, and line height. The createPage method generates a canvas element for each page, and renderPagePaddingIndicators draws the four corner guides.

Row computation is performed in computeRows , where a temporary canvas measures each character using ctx.measureText . The method records width, height (including actualBoundingBoxAscent and actualBoundingBoxDescent ), original height, and maximum descent to support vertical centering and line‑height scaling.

During rendering, renderRow draws background, underline, strikethrough, and the text itself, adjusting the y‑coordinate to vertically center characters based on the calculated heights. The cursor is rendered as a thin div positioned with setCursor , and its blinking is handled by a timer.

Keyboard interaction is managed through a hidden textarea for focus. The onInput handler inserts typed characters (preserving the style of the surrounding element), while onKeydown processes backspace ( delete ) and Enter ( newLine ) to modify the JSON data and re‑render the view.

Selection is implemented by tracking mouse down, move, and up events. The current selection range is stored in this.range , and renderRow paints a semi‑transparent rectangle over elements whose indices fall within the computed range, using rangeColor and rangeOpacity . The editor also hides the cursor when a selection exists and clears the selection when the cursor is set.

Finally, the article notes that while basic editing, styling, cursor, and selection are functional, many advanced features such as copy/paste, arrow‑key navigation, drag‑and‑drop, images, tables, and links remain to be implemented for a production‑ready editor.