Concatenation using for loop Javascript

The easy way is by using Array.reduce().

const checkboxes = document.querySelectorAll("input[type=checkbox]");

const result = Array
  .from(checkboxes)
  .reduce((acc, checkbox) => acc + Number(checkbox.value), 0);

console.log(result);

<form class="jumbotron form3">
  <h5>EXERCICE 7</h5>
  <p>Veuillez cocher trois cases:</p>
  <hr>
  <input type="checkbox" name="1" value="-8"> Case 1
  <br><input type="checkbox" name="2" value="5"> Case 2
  <br><input type="checkbox" name="3" value="3"> Case 3
  <br><input type="checkbox" name="4" value="10"> Case 4
  <br><input type="checkbox" name="5" value="0"> Case 5
  <br><input type="checkbox" name="6" value="-1"> Case 6
  <br><input type="checkbox" name="7" value="-7"> Case 7
  <br><input type="checkbox" name="8" value="15"> Case 8
  <br><input type="checkbox" name="9" value="8"> Case 9
  <br><input type="checkbox" name="10" value="3"> Case 10
  <hr>
  <input type="button" class="btn btn-warning" value="Calculer le score du jeu" onClick="calculerScore()">
  <input type="button" class="btn btn-info" value="Reset">
  <br><br>
  <input type="text" name="score">
</form>

You can access your dom elements using querySelector method. We need querySelectorAll since we want to get all matching elements.

Main Steps:

• Fetches only cases that are checked.
• Takes their values.
• Sums these values.
• Sets final result on score input tag.

Javascript code

const calculerScore = () => {

        // get ONLY checked boxes
        const allCheckedBoxes = window.document.querySelectorAll('input[type=checkbox]:checked');    // returns Nodelist object
        console.log('allCheckedBoxes: ', allCheckedBoxes);

        // take all checked boxes values in seperate array
        const checkedBoxesValues = [...allCheckedBoxes].map(box => {
            return parseInt(box.value);
        })
        console.log("checkedBoxesValues:", checkedBoxesValues)
        
        // sum all values
        let resultat = 0;
        checkedBoxesValues.forEach(value => {
            resultat += value;
        });
        console.log("resultat: ",resultat);
        
        // finally set value of score input tag
        window.document.querySelector('input[name="score"]').value = resultat;
    }

I have added consoles and comments for understanding.

There is a simpler way to do it with reduce as @Invizi mentioned, However, I have omitted it for now perceiving your understanding of JS.

Let me know if it helped.

Firstly , you need to optimize that code , don't hard code everything , add a single tag in a loop and render that.( Just a Suggestion )

Secondly , why even access all the input elements by accessing the DOM , you could easily get them through the Form you appended on the top. When you attach a Form to a group of Input Tags , it stores those instances in the Form itself , simply get the Form ( you could attach an ID to that ) and access all the inputs through that instance.