JSFiddle - React, Tailwind, and code Playground

HTML

<label for="value">Enter Value: </label>
            <input id="value1" class="value1" type="number" title="value field" placeholder="e.g. 1000">
            <button type="button" id="solution1_btn" title="solution 1 button">Enter</button>
            <input id="result1" class="result1" type="number" disabled>

JavaScript

// click button
var solution1 = document.getElementById("solution1_btn");

// user entered value
//var value1 = document.getElementById("value1");

//console.log(value1);

// result input field
var result1 = document.getElementById("result1");
var value1 = document.getElementById("value1");

function multiples(event) {

    var fix = parseInt(value1.value);
    // declare sum assign it value 0   
    var sum = 0;
    //  declare a for loop that iterates while the counter "i" is less than fix   
    for (var i = 0; i < fix; i++) {
        // if statement test whether the division remainder of 3 and 5 are equal to 0
        if (i % 3 == 0 && i % 5 === 0) {
            // assigns the value of i to sum 
            sum += i;
            var newSum;
            newSum = sum;
        };
    };
    // returns the value of sum from the function callback argument 1000 etc.
    result1.value = newSum
};



solution1.onclick = multiples;