const
bruce
=
{
name
:
"Bruce"
};
const
madeline
=
{
name
:
"Madeline"
};
// this function isn't associated with any object, yet
// it's using 'this'!
function
greet
() {
return
`Hello, I'm
${
this
.
name
}
!`
;
}
greet
();
// "Hello, I'm !" - 'this' not bound
greet
.
call
(
bruce
);
// "Hello, I'm Bruce!" - 'this' bound to 'bruce'
greet
.
call
(
madeline
);
// "Hello, I'm Madeline!" - 'this' bound to 'madeline'
You can see that
call
allows us to call a function as if it were a method by providing
it an object to bind
this
to. The first argument to
call
is the value you want
this
bound to, and any remaining arguments become arguments to the function you’re
calling:
function
update
(
birthYear
,
occupation
) {
this
.
birthYear
=
birthYear
;
this
.
occupation
=
occupation
;
}
update
.
call
(
bruce
,
1949
,
'singer'
);
// bruce is now { name: "Bruce", birthYear: 1949,
// occupation: "singer" }
update
.
call
(
madeline
,
1942
,
'actress'
);
// madeline is now { name: "Madeline", birthYear: 1942,
// occupation: "actress" }
apply
is identical to
call
except the way it handles function arguments.
call
takes
arguments directly, just like a normal function.
apply
takes its arguments as an array:
update
.
apply
(
bruce
, [
1955
,
"actor"
]);
// bruce is now { name: "Bruce", birthYear: 1955,
// occupation: "actor" }
update
.
apply
(
madeline
, [
1918
,
"writer"
]);
// madeline is now { name: "Madeline", birthYear: 1918,
// occupation: "writer" }
apply
is useful if you’ve got an array and you want to use its values as arguments to a
function. The classic example is finding the minimum or maximum number in an
array. The built-in
Math.min
and
Math.max
functions take any number of arguments
and return the minimum or maximum, respectively. We can use
apply
to use these
functions with an existing array:
const
arr
=
[
2
,
3
,
-
5
,
15
,
7
];
Math
.
min
.
apply
(
null
,
arr
);
// -5
Math
.
max
.
apply
(
null
,
arr
);
// 15
Note that we simply pass
null
in for the value of
this
. That’s because
Math.min
and
Math.max
don’t use
this
at all; it doesn’t matter what we pass in here.
call, apply, and bind | 115
