Can I declare default parameter like
function myFunc( a, b=0)
{
// b is my optional parameter
}
in JavaScript?
Add a comment
|
2 Answers
With ES6: This is now part of the language:
function myFunc(a, b = 0) {
// function body
}
Please keep in mind that ES6 checks the values against undefined and not against truthy-ness (so only real undefined values get the default value - falsy values like null will not default).
With ES5:
function myFunc(a,b) {
b = b || 0;
// b will be set either to b or to 0.
}
This works as long as all values you explicitly pass in are truthy.
Values that are not truthy as per MiniGod's comment: null, undefined, 0, false, ''
It's pretty common to see JavaScript libraries to do a bunch of checks on optional inputs before the function actually starts.
11 Comments
MiniGod
null, undefined, 0, false, '', NaN will all get the default value.MiniGod
If you only want the default value if b is omitted, use
if (typeof b === 'undefined') b = 0;
kalu
@MiniGod will this throw
b into the global namespace when b is omitted?
Tigraine
@kalu no because you have defined it in the local scope (the function). It is declared and scoped at the function level but will be undefined if not called from outside. So no global scope assignment happens here.
Tigraine
Why? I set the default value of 0. If you want another default value just change that to
b = b || 'foo'. |
Update
With ES6, this is possible in exactly the manner you have described; a detailed description can be found in the documentation.
Old answer
Default parameters in JavaScript can be implemented in mainly two ways:
function myfunc(a, b)
{
// use this if you specifically want to know if b was passed
if (b === undefined) {
// b was not passed
}
// use this if you know that a truthy value comparison will be enough
if (b) {
// b was passed and has truthy value
} else {
// b was not passed or has falsy value
}
// use this to set b to a default value (using truthy comparison)
b = b || "default value";
}
The expression b || "default value" evaluates the value AND existence of b and returns the value of "default value" if b either doesn't exist or is falsy.
Alternative declaration:
function myfunc(a)
{
var b;
// use this to determine whether b was passed or not
if (arguments.length == 1) {
// b was not passed
} else {
b = arguments[1]; // take second argument
}
}
The special "array" arguments is available inside the function; it contains all the arguments, starting from index 0 to N - 1 (where N is the number of arguments passed).
This is typically used to support an unknown number of optional parameters (of the same type); however, stating the expected arguments is preferred!
Further considerations
Although undefined is not writable since ES5, some browsers are known to not enforce this. There are two alternatives you could use if you're worried about this:
b === void 0;
typeof b === 'undefined'; // also works for undeclared variables
