JSFiddle - React, Tailwind, and code Playground

JavaScript

var a = [1, 2, 5, 9];
var b = [2, 5, 100, 0, 41];
var c = 0;

for (let i = 0; i < (a.length > b.length ? a.length : b.length); i++) {
		if (i in b && a.indexOf(b[i]) < 0) { 
			c++; 
				for (let j = b.length-1, r = 0; j >= 0; j--) {
						 if (b[i] === b[j]) r++;
             if (r > 1) {c--; break}
				}
		}
		if (i in a && b.indexOf(a[i]) < 0) { 
			c++; 
				for (let j = a.length-1, r = 0; j >= 0; j--) {
						 if (a[i] === a[j]) r++;
             if (r > 1) {c--; break}
				}
		}
}

alert(c)