Edit in JSFiddle

var array = [1, 2, 3], 
    valor = 1;

for(i = 0; i < array.length; i++){
    document.body.innerHTML += array[i] + "<br />";
    if(valor==array[i]) alert("coinciden");
}       
 
var index = array.indexOf(valor);
alert(index);
if(index != -1) array.splice(index, 1);
document.body.innerHTML = array.join("<br />");