JSFiddle - React, Tailwind, and code Playground

HTML

<input text="text" value="4" id="input5"/>
<input text="text" value="4" id="input6"/>
<br/>
<input text="text" value="2" id="input7"/>
<input text="text" value="4" id="input8"/>

JavaScript

var els = [
    [
        document.getElementById('input5'),
        document.getElementById('input6')
    ],
    [
        document.getElementById('input7'),
        document.getElementById('input8')
    ]
];

var result = [];

for (var i = 0; i < els.length; i++) {
    result[result.length] = els[0][i].value - els[1][i].value;
}

alert(result.join(' '));
console.log(result);