One or both of the variables is a string instead of a number. This makes the + do string concatenation.
'2' + 2 === '22'; // true
2 + 2 === 4; // true
The other arithmetic operators / * - will perform a toNumber conversion on the string(s).
'3' * '5' === 15; // true
A quick way to convert a string to a number is to use the unary + operator.
+'2' + 2 === 4; // true
...or with your variables:
+x + +y
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