public class Test {
    public String variable1 {get; set;}
    public String variable2 {get; set;}

    public void mTest(){
        System.debug(variable1 + ' ' + variable2);
    }
}

<apex:page controller="Test">
<apex:form>
    <apex:actionFunction name="JS" action="{!mTest}">
        <apex:param assignTo="{!variable1}" name="var1" value=""/>
        <apex:param assignTo="{!variable2}" name="var2" value=""/>
    </apex:actionFunction>
</apex:form>

<form>
    <input type="text" id="input1"/><br/>
    <input type="text" id="input2"/><br/>
    <button type="button" onclick="doJS()">
        Click
    </button>
</form>

<script>
    function doJS(){
        let x = document.getElementById('input1').value;
        let y = document.getElementById('input2').value;

        JS(x, y);
    }
</script>