Or assign a function to an object property:
const
o
=
{};
o
.
f
=
getGreeting
;
o
.
f
();
// "Hello, World!"
Or even add a function to an array:
const
arr
=
[
1
,
2
,
3
];
arr
[
1
]
=
getGreeting
;
// arr is now [1, function getGreeting(), 2]
arr
[
1
]();
// "Hello, World!"
This last example should make clear the role of the parentheses: if JavaScript encoun‐
ters parentheses that follow a value, the value is assumed to be a function, and that
function is called. In the preceding example,
arr[1]
is an expression that resolves to a
value. That value is followed by parentheses, which is a signal to JavaScript that the
value is a function and should be called.
If you try to add parentheses to a value that is not a function, you
will get an error. For example, "whoops"() will result in the error
TypeError: "whoops" is not a function
.
Function Arguments
We’ve now seen how to call functions and get values out of them; how about getting
information into them? The primary mechanism to pass information to a function
call is function arguments (sometimes called parameters). Arguments are like vari‐
ables that don’t exist until the function is called. Let’s consider a function that takes
two numeric arguments and returns the average of those numbers:
function
avg
(
a
,
b
) {
return
(
a
+
b
)
/
2
;
}
In this function declaration,
a
and
b
are called formal arguments. When a function is
called, formal arguments receive values and become actual arguments:
avg
(
5
,
10
);
// 7.5
In this example, the formal arguments
a
and
b
receive the values
5
and
10
, and
become actual arguments (which are very much like variables, but specific to the
function body).
One thing that beginners often stumble over is that the arguments exist only in the
function, even if they have the same name as variables outside of the function.
Consider:
Function Arguments | 105
