ReadAndEditAll.js

/**
  These scripts will only work in Salesforce classic. 
**/

//For Edit access column 
var arrayEdit = document.getElementsByClassName('displayedCol');
for(i=0;i<arrayEdit.length;i++){
  if(arrayEdit[i]['firstElementChild']!= undefined)
    arrayEdit[i]['firstElementChild'].checked = true;
}

//-------------------------------------

//For Read access column
var arrayRead = document.getElementsByClassName('readonlyCol');
for(i=0;i<arrayRead.length;i++){
  if(arrayRead[i]['firstElementChild']!= undefined)
    arrayRead[i]['firstElementChild'].checked = true;
}