The y1 read-only property of the SVGLineElement interface describes the start of the SVG line along the y-axis as an SVGAnimatedLength. It reflects the <line> element's y1 geometric attribute.
The attribute value is a <length>, <percentage>, or <number>. The numeric value of the SVGAnimatedLength.baseVal is that start position as a length along the y-axis in user coordinate system units.
Given the following SVG:
<svg viewBox="0 0 300 200" xmlns="http://www.w3.org/2000/svg">
<line x1="20" y1="30" x2="40" y2="50" stroke="blue" stroke-width="2" />
<line x1="15%" y1="5%" x2="30%" y2="60%" stroke="red" stroke-width="4" />
</svg>We can access the computed values of the y1 attributes:
const lines = document.querySelectorAll("line");
const y1Pos0 = lines[0].y1;
const y1Pos1 = lines[1].y1;
console.dir(y1Pos0.baseVal.value); // output: 30 (the value of `y1`)
console.dir(y1Pos1.baseVal.value); // output: 10 (5% of 200)