fitTextInsideBox(document.querySelector('.text'), document.querySelector('.bounding-box'));
function fitTextInsideBox(text, box) {
// bounding box parameters
var boxPosX = 20
var boxPosY = 20;
var boxWidth = 500
var boxHeight = 500;
box.style.position = 'absolute';
box.style.left = `${boxPosX}px`;
box.style.top = `${boxPosY}px`;
box.style.width = `${boxWidth}px`;
box.style.height = `${boxHeight}px`;
text.style.position = 'absolute';
text.style.left = `${boxPosX}px`;
text.style.top = `${boxPosY}px`;
text.style.width = `${boxWidth}px`;
var fontSize = 0;
increaseFontSize(text, box);
function increaseFontSize(text, box) {
fontSize++;
text.style.fontSize = `${fontSize}px`;
var textHeight = text.getBoundingClientRect().height;
if (textHeight <= boxHeight) {
increaseFontSize(text, box);
}
else {
fontSize--;
text.style.fontSize = `${fontSize}px`;
}
}
}
body,
html {
margin: 0;
padding: 0;
}
.bounding-box {
border: 2px solid red;
box-sizing: border-box;
}
<div class="bounding-box"></div>
<span class="text">Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt
ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea
rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit
amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam
erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no
sea takimata sanctus est Lorem ipsum dolor sit amet.</span>