Solution:

goog.net.XhrIo.send(url, undefined, undefined, undefined, {'Cache-Control': 'no-cache'})

You should use undefined instead of optional parameter you want to skip, because this 100% simulates the default value for optional parameters in JavaScript.

Small example:

myfunc(param);//is equivalent tomyfunc(param, undefined, undefined, undefined);

Strong recommendation: use JSON if you have a lot of parameters, and you can have optional parameters in the middle of the parameters list. Look how this is done in jQuery.

Short answer

The safest bet is undefined, and should work almost ubiquitously. Ultimately, though, you cannot trick the function being called into thinking you truly omitted a parameter.

If you find yourself leaning towards using null just because it's shorter, consider declaring a variable named _ as a nice shorthand for undefined:

(function() { // First line of every script file    "use strict";    var _ = undefined; // For shorthand    // ...    aFunction(a, _, c);    // ...})(); // Last line of every script

Details

First, know that:

• typeof undefined evaluates to "undefined"
• typeof null evaluates to "object"

So suppose a function takes an argument that it expects to be of type "number". If you provide null as a value, you're giving it an "object". The semantics are off.¹

As developers continue to write increasingly robust javascript code, there's an increasing chance that the functions you call explicitly check a parameter's value for undefined as opposed to the classic if (aParam) {...}. You'll be on shaky ground if you continue to use null interchangeably with undefined just because they both happen to coerce to false.

Be aware, though, that it is in fact possible for a function to tell if a parameter was actually omitted (versus being set to undefined):

f(undefined); // Second param omittedfunction f(a, b) {    // Both a and b will evaluate to undefined when used in an expression    console.log(a); // undefined    console.log(b); // undefined    // But...    console.log("0" in arguments); // true    console.log("1" in arguments); // false}

Footnotes

1. While undefined also isn't of type "number", it's whole job is to be a type that isn't really a type. That's why it's the value assumed by uninitialized variables, and the default return value for functions.

Just pass null as parameter value.

Added: you also can skip all consequent optional parameters after the last that you want to pass real value (in this case you may skip opt_timeoutInterval parameter at all)