Slidebar which can be adjusted manually

javascript

The issue is that textContent returns string and using + operator will try to concatenate the values. So you can parse it. But I would recommend directly connecting your range with displayed value and only alter the range using the buttons or change them together for example:

   function rangeSlide1(value) {
        document.getElementById('rangeValue1').innerHTML = value;
      }
    
      function moins1() {
        let range = document.querySelector('.range')
        if (range.value != 0) {
          let newValue = range.value - 1
          document.getElementById('rangeValue1').innerHTML = newValue
          range.value = newValue
        }
      }
    
      function plus1() {
        let range = document.querySelector('.range')
        if (range.value < 99) {
          let newValue = Number(range.value) + 1
          document.getElementById('rangeValue1').innerHTML = newValue
          range.value = newValue
       }
      }

    <div>
      <button id="moins1" onclick="moins1()">MOINS 1</button>
      <span class="unselectable rangeValue1" id="rangeValue1">RR</span>
      <Input class="range" type="range" name="BarreRR" value="0" min="0" max="99" onChange="rangeSlide1(this.value)" onmousemove="rangeSlide1(this.value)"></Input>
      <button id="plus1" onclick="plus1()">PLUS 1</button>
    </div>

Considering your range is limited from 0 to 100 I added condition not to go under 0

Slidebar which can be adjusted manually

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