You can declare a function with optional parameter using Logical OR operator (‘||’) or Assignment operator (“=”), etc in JavaScript.
JavaScript function optional parameter can do it in 3 ways:-
- Undefined property
- OR (||) operator
- The assignment operator (“=”)
Declare optional function parameters in JavaScript Example
Let’s see all method examples in HTML:-
Using undefined property
Undefined property indicates that a value is not assigned to a variable.
<!DOCTYPE html>
<html>
<body>
<script>
function browseBlog(blogURL, type) {
if(blogURL === undefined)
{
blogURL = "DefaultURL";
}
if(type === undefined)
{
type = "DefaultType";
}
alert(blogURL);
alert(blogType);
}
//Test cases
browseBlog("www.abc.com", "EyeHunts");
browseBlog("www.abc.com");
</script>
</body>
</html>Output:
OR (||) operator
In the example the optional parameter is ‘b’:
The optional parameter is Logically OR with the default value within the body of the function and should always come at the end of the parameter list.
Pre ES2015,
<!DOCTYPE html>
<html>
<body>
<script>
function check(a, b) {
b = b || 0;
console.log(a, b);
}
// Test cases
check(1, 2);
check(10);
</script>
</body>
</html>Output:
The assignment operator (“=”)
The optional variable is assigned the default value in the declaration statement itself and should end on the parameter list.
From ES6/ES2015, default parameters are in the language specification.
<script>
function check(a, b = 0) {
console.log(a, b);
}
// Test cases
check(1, 2);
check(10);
</script> Output:
1 2
10 0
Do comment if you have any doubts and suggestion on this JS function topic.
Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version