I'm hoping to be able to find the height of the baseline of a piece of text in a div using javascript.
I can't use a predefined font, because many of my users will be using a larger font setting in their browser.
How can I do this in javascript?
I made a tiny jQuery plugin for that. The principle is simple:
In a container, insert 2 inline elements with the same content but one styled very small .
and the other very big A
.
Then, since there are vertical-align:baseline
by default, the baseline is given as follow:
^ +----+ ^
| | +-+| | top
height | |.|A|| v
| | +-+|
v +----+
=======================
baseline = top / height
=======================
Here is the plugin in coffeescript (JS here):
$ = @jQuery ? require 'jQuery'
detectBaseline = (el = 'body') ->
$container = $('<div style="visibility:hidden;"/>')
$smallA = $('<span style="font-size:0;">A</span>')
$bigA = $('<span style="font-size:999px;">A</span>')
$container
.append($smallA).append($bigA)
.appendTo(el);
setTimeout (-> $container.remove()), 10
$smallA.position().top / $bigA.height()
$.fn.baseline = ->
detectBaseline(@get(0))
then, smoke it with:
$('body').baseline()
// or whatever selector:
$('#foo').baseline()
--
Give it a try at: http://bl.ocks.org/3157389
Following the discovery by Alan Stearns (http://blogs.adobe.com/webplatform/2014/08/13/one-weird-trick-to-baseline-align-text/) that inline-block elements change the baseline alignment of other inline elements, I put together a demo which I found more accurate than https://stackoverflow.com/a/11615439/67190, and actually more simple to derive a value in JavaScript: http://codepen.io/georgecrawford/pen/gbaJWJ. Hope it's useful.
<div>
<span class="letter">T</span>
<span class="strut"></span>
<div>
div {
width: 100px;
height: 100px;
border: thin black solid;
}
.letter {
font-size: 100px;
line-height: 0px;
background-color: #9BBCE3;
}
.strut {
display: inline-block;
height: 100px;
}
Vanilla JS, no font info required tested on Chrome, Firefox and Edge :
function getBaselineHeight(el){
let bottomY = el.getBoundingClientRect().bottom;//viewport pos of bottom of el
let baselineLocator = document.createElement("img"); // needs to be an inline element without a baseline (so its bottom (not baseline) is used for alignment)
el.appendChild(baselineLocator);
baselineLocator.style.verticalAlign = 'baseline' ; // aligns bottom of baselineLocator with baseline of el
let baseLineY = baselineLocator.getBoundingClientRect().bottom;//viewport pos of baseline
el.removeChild(baselineLocator);
return (bottomY - baseLineY) ;
}
function positionLine(){
let line = document.getElementById("line");
let lorem = document.getElementById("lorem");
let baselineHeight = getBaselineHeight(lorem);
let newLinePos = lorem.getBoundingClientRect().bottom - baselineHeight ;
line.style.top = newLinePos + "px" ;
}
#lorem{
background-color: lightblue;
}
#line{
position:absolute;
left : 0 ;
height:1px;
width:100%
}
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" rel="stylesheet"/>
<span id="lorem">Lorem ipsum dolor sit amet, consectetur adipiscing elit..</span>
<br><br>
<button class="btn btn-primary" onclick="positionLine();">position line on baseline</button>
<br><br>
<svg id="line">
<line x1="0" y1="0" x2="100%" y2="0" style="stroke:rgb(255,0,0);stroke-width:1"></line>
</svg>
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