many local variables declaration vs one global?

i'm just new javascript biginner, 
If i need to reuse many times (lets say 20 times) the same variable, from a performance point of view,
should i declare it one time as global , or declare it many times in each function context in local scope ( as i heard about ) ?
whats case bellow is better ?
caseA :
var myvar = document.getElementById('case'); //global

function one(){
myvar.innerHTML = 'newb';
...
}
function two(){
myvar.value = '3';
...
}

caseB :
function one(){
var myvar = document.getElementById('case'); //local
myvar.innerHTML = 'newb';
...
}
function two(){
var myvar = document.getElementById('case'); //lobal 
myvar.value = '3';
...
}

caseC :
var myvar = document.getElementById('case'); //global

function one(){
var myvar = document.getElementById('case'); //local
myvar.innerHTML = 'newb';
...
}
function two(){
var myvar = document.getElementById('case'); //lobal 
myvar.value = '3';
...
}

thx for advises,
gui

Wrap case A inside a closure
(function() {
  var myvar = document.getElementById('case');

  function one(){
    myvar.innerHTML = 'newb';
    ...
  }

  function two(){
    myvar.value = '3';
    ...
  }
})()

now nothing is in the global scope but you only access the DOM once.