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Calculate line-height of an HTML element (IE6 compatible)


Calculate line-height of an HTML element (IE6 compatible)

This was created for provide a well-tested module for calculating line-height in pixels for trunkata, a line-based truncation library for HTML.

line-height is available via the following:

For npm and component, you can load it in as follows:

var lineHeight = require('line-height');

For bower and http, you can use vanilla JS

<script src="components/line-height.js"></script>
window.lineHeight; // `line-height` is defined on `window` in camelCase

or you can use AMD

require(['line-height'], funtion (lineHeight) { /* code */ });

or CommonJS syntax (see npm/component section).

Once you have the module loaded, you can get the line-height of any node in the DOM.

// Calculate the `line-height` of the body 
lineHeight(document.body); // 19 
// Calculate the `line-height` of an h2 
var h2 = document.createElement('h2');
lineHeight(h2); // 29 
// Calculate how many lines tall an element is 
var div = document.createElement('div');
div.innerHTML = '<p>1</p><p>2</p>';
(lineHeight(div) / div.offsetHeight); // 2, how trunkata performs its calculations 

line-height provides a single function.

 * Calculate the `line-height` of a given node
 * @param {HTMLElement} node Element to calculate line height of. Must be in the DOM.
 * @returns {Number} `line-height` of the element in pixels

In a large amount of browsers, the computed style for an element's line-height is normal by default.

If it is specified by any other means (e.g. ancestor has a line-height or the element has a line-height specified), it is either a CSS length.

To solve this problem, we create a vanilla element of the same nodeName (e.g. h2 if it is an h2), apply the original element's font-size, and return the element offsetHeight. This is the height of 1 line of the element (i.e. line-height).

In most browsers, when the line-height is specified in pt, pc, in, cm or mm, the computedStyle value is in the same unit.

To solve this problem, we use the standard ratios of conversion to pixels to make a conversion to pixels.

  • 3pt to 4px
  • 1pc to 16px
  • 1in to 96px
  • 2.54cm to 96px
  • 25.4mm to 96px

In IE6, numeric font-sizes (e.g. font-size: 2.3) are returned without a unit.

To solve this problem, we treat this number as an em since it is relative as well. To do that, we set the element's style to "numeric value" + "em", compute and save the font-size, remove the temporary style. This conversion gives us the unit in pt which we know how to deal with from before.

In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint via grunt and test via npm test.

Copyright (c) 2013 Todd Wolfson

Licensed under the MIT license.