Skipping optional function parameters in JavaScript
Solution 1
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 to
myfunc(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.
Solution 2
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 omitted
function 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
- 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.
Solution 3
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)
Dan
Updated on July 05, 2022Comments
-
Dan almost 2 years
Could you please point me to the nice way of skipping optional parameters in JavaScript.
For example, I want to throw away all
opt_
parameters here:goog.net.XhrIo.send(url, opt_callback, opt_method, opt_content, {'Cache-Control': 'no-cache'}, opt_timeoutInterval)
-
Dan over 12 yearsAre you quite sure there is no ` === undefined` checking inside?
-
nnnnnn over 12 years@Dan, you phrased your question as a generic JavaScript question with that code as an example. If you need help specifically with how
goog.net.XhrIo.send()
works please rephrase the question. -
Dan over 12 yearsThank you very much @nnnnnn, but this is really a generic JavaScript question and the answer should work in any case. If there is no generic solution, please post it in your answer.
-
Yuriy Rozhovetskiy over 12 years@Dan, I can't be sure, how developer who's implement the
goog.net.XhrIo.send
method check for optional parameters. He's may check forparam === undefined
ortypeof param === "undefined"
orparam === null
or event justif(param)
. But when you pass null value it's a sign that you're realizing that this parameter is optional and explicitelyy tell that you don't want to pass any meaning value. -
nnnnnn over 12 yearsYou could pass undefined instead of null, it could be more likely to work in more cases, but I'm not sure there's an answer that will work in any case. If I were writing a function with optional params I'd probably check with
==null
to allow for undefined or null (unless null is a legit value for some reason), but I might check the length of thearguments
object, and I've seen other code that checked with===null
so... -
hansvb over 12 years+1. I don't know what this does exactly (same as
undefined
?) but it sure looks best for "skipping optional parameters". -
Dan about 11 yearsI have found that you can never be sure what to pass instead of "skipped" parameter. Undefined? Null? 0? Empty string? Empty object? Array? If behaviour of omittting the parameter in not docummented, or there is no default parameter, you are in great trouble. You can't sleep well if you update the external library. Only god and maybe library developer can know what will happen if you pass anything unexpected instead of the required parameter. Generaly if this behaviour is not documented, the bugs are on you
-
Anonymous about 7 yearsTHis still works in 2017 on the latest google chrome version so I'm upvoting.
-
MarekJ47 almost 7 yearsHi, this works well, unless the function uses arguments.length. See this example: argl = function() { return arguments.length; } argl(1) => 1 argl(1, undefined, undefined) => 3 argl(1, undefined, 2) => 3
-
Newlukai about 5 yearsThe npm module oracledb makes heavy use of arguments.length. Mostly to decide if a callback is given. Unfortunately,
undefined
is not an option to call these methods. One needs introduce anif
to choose the right parameter list length wtf -
auerbachb over 3 yearsonce null is evaluated it may cause the function to fail, thus,
undefined
should be included