用法：with (<对象>) <语句>;

The with() statement in JavaScript is identical to the With statement found in Visual Basic. Using with(), you can reduce object references and make your code more readable. Surprisingly, it is possible to have nested with() statements. For example, this code:

function foo(){
var x = document.forms[0].elements[0].value;
var y = document.forms[0].elements[1].value;
var z = document.forms[0].elements[2].options[document.forms[0].elements[2].selectedIndex].text;
}

could be rewritten as:

function foo(){
with(document.forms[0]){
var x = elements[0].value;
var y = elements[1].value;
with(elements[2]){
var z = options[selectedIndex].text
}
}
}

another example:

x = Math.cos(3 * Math.PI) + Math.sin(Math.LN10);
y = Math.tan(14 * Math.E);

could be rewritten as:

with (Math) {
x = cos(3 * PI) + sin(LN10);
y = tan(14 * E);
}