I have a class of dropdowns -- .selMapField -- and I want to select the dropdown that has a specific value. For instance:
const myattr=$(`.selMapField[value="MYSKU"]`).attr(`x`);
Use jQuery's .filter() to find the elements by value
const myattr = $(".selMapField").filter((_, { value }) =>
value === "MYSKU").attr("x")
console.log(myattr)<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.slim.min.js"></script>
<select class="selMapField" x="A"><option selected>FOO</option></select>
<select class="selMapField" x="B"><option selected>MYSKU</option></select>
<select class="selMapField" x="C"><option selected>BAR</option></select>As always, you might not need jQuery
const myattr = Array.from(
document.querySelectorAll(".selMapField")
).find(({ value }) => value === "MYSKU")?.getAttribute("x")
console.log(myattr)<select class="selMapField" x="A"><option selected>FOO</option></select>
<select class="selMapField" x="B"><option selected>MYSKU</option></select>
<select class="selMapField" x="C"><option selected>BAR</option></select>