How to select all in CheckBox? _JavaScript

There are many CheckBoxes in a form. How can I select one and select all of them?
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function selectAll(checkBoxName){
var elms = document.getElementsByName(checkBoxName);
if (elms.length< 1) return; type=="checkbox")
allChecked &= elms[i].checked;
for (var i=0;i if (elms[i].type=="checkbox"){
                                                                                                                                                                                       [i].checked = !allChecked;
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Select all< input type=checkbox value="Check All" onclick="mm(this)">











Select all: