JavaScript to achieve full or reverse selection effect in form

<script>
        //Example of selecting all or unselecting all in a form: Note: If the checked attribute value of a checkbox is true, it is selected, and if it is false, it is unselected var j_cbAll = document.getElementById('j_cbAll'); //Get the select all button var j_tbs = document.getElementById('j_tb').getElementsByTagName('input'); //Get all input checkboxes //Register events //1. Event of the select all button j_cbAll.onclick = function () {
            // this.checked can get the status of the current checkbox and return a value of true or false
            for (var i = 0; i < j_tbs.length; i++) {
                j_tbs[i].checked = this.checked; // Make the checked value of each checkbox in tbody equal to the checked value of the current select all button, so their states will be consistent}
        }
        // 2. Events of the checkboxes in tbody: If all the checkboxes below are selected, the Select All button will also be selected. If at least one is not selected, the Select All button will also be unselected. Note: Every time a checkbox in tbody is clicked, a check needs to be made to see if all the checkboxes are selected for (var i = 0; i < j_tbs.length; i++) {
            j_tbs[i].onclick = function () {
                // flag is used to control whether the select all button is selected var flag = true;
                // Each time you click the checkbox below, you need to loop and check if all 4 checkboxes are selected for (var i = 0; i < j_tbs.length; i++) {
                    console.log('---'+j_tbs[i].checked);
                    if (j_tbs[i].checked == false) { // If a button is not selected, execute the internal code to modify the flag value to false
                        flag = false;
                        break; // Exit the inner for loop, which can improve execution efficiency, because as long as one is not selected, the rest do not need to be judged.}
                }
                j_cbAll.checked = flag; //After the loop is finished, set the flag value to the select all button}
        }
</script>