How Can I Calculate Text Width in JavaScript Using the Canvas Method?

How to Calculate Text Width with JavaScript

Calculating the width of a string using JavaScript is not immediately straightforward, especially when using a non-monospace typeface. Here, we explore a solution using the Canvas.measureText method in HTML 5.

HTML 5 Canvas Method for Text Width Calculation

To use Canvas.measureText, we create a canvas context and assign it the desired font style. Then, we use the measureText method to measure the width of the specified text.

function getTextWidth(text, font) {
  const canvas = getTextWidth.canvas || (getTextWidth.canvas = document.createElement("canvas"));
  const context = canvas.getContext("2d");
  context.font = font;
  const metrics = context.measureText(text);
  return metrics.width;
}

For example, to calculate the width of "hello there!" in "bold 12pt arial" font, we can do:

console.log(getTextWidth("hello there!", "bold 12pt arial"));  // close to 86

Using CSS Font Styles

If we want to use the font-size of a specific element, we can use the getCanvasFont utility function:

const fontSize = getTextWidth(text, getCanvasFont(myEl));

Advantages of Canvas Method

Compared to DOM-based methods, the Canvas method has several advantages:

• Conciseness and safety: It does not change global state or the DOM.
• Customization: It allows further customization of text properties like textAlign and textBaseline.

Notes

• When adding text to the DOM, consider padding, margin, and border effects.
• On some browsers, the Canvas method provides sub-pixel accuracy (floating point result), while on others it is only accurate to integers.
• In Firefox, the Canvas method is significantly faster than DOM-based methods.

Performance Comparison

A jsperf benchmark shows that the Canvas method and a caching DOM-based method have similar performance, except in Firefox where the Canvas method is notably faster.

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