[ruslan_mart]

А если по хорошему, то:

<input type="checkbox" name="bigbox">

var checkbox = document.getElementsByName('bigbox')[0];

checkbox.addEventListener('click', function() {
    this.form.weight.disabled = this.form.units.disabled = !this.checked;
    
});

document.addEventListener('keypress', function(e) {
    checkbox.click();
});