html

<svg viewBox="0 0 200 200" xmlns="http://www.w3.org/2000/svg">
  <defs>
    <!-- First pattern -->
    <pattern
      id="pattern1"
      y="50"
      width="20"
      height="20"
      patternUnits="userSpaceOnUse">
      <circle cx="10" cy="10" r="10" fill="blue" />
    </pattern>
    <!-- Second pattern -->
    <pattern
      id="pattern2"
      y="100"
      width="20"
      height="20"
      patternUnits="userSpaceOnUse">
      <circle cx="10" cy="10" r="10" fill="red" />
    </pattern>
  </defs>
  <!-- Using the patterns -->
  <rect x="0" y="0" width="100" height="100" fill="url(#pattern1)" />
  <rect x="100" y="0" width="100" height="100" fill="url(#pattern2)" />
</svg>

js

const patterns = document.querySelectorAll("pattern");

console.log(patterns[0].y.baseVal.value); // output: 50
console.log(patterns[1].y.baseVal.value); // output: 100