Check all Checkboxes in Page via Developer Tools

From Console Dev Tools (F12) you can use query selector as you use in javascript or jQuery code.

'$$' - means select all items. If you use '$' instead you will get only first item.

So in order to select all checkboxes you can do following

$$('input').map(i => i.checked = true)

or

$$('input[type="checkbox"').map(i => i.checked = true)

(function() {
    var aa= document.getElementsByTagName("input");
    for (var i =0; i < aa.length; i++){
        if (aa[i].type == 'checkbox')
            aa[i].checked = true;
    }
})()

With up to date browsers can use document.querySelectorAll

(function() {
    var aa = document.querySelectorAll("input[type=checkbox]");
    for (var i = 0; i < aa.length; i++){
        aa[i].checked = true;
    }
})()