Array Searching
If you want to find something in an array, you have a few choices. We’ll start with the
humble
indexOf
, which has been available in JavaScript for some time.
indexOf
sim‐
ply returns the index of the first element it finds that is strictly equal to what you’re
looking for (there is a corresponding
lastIndexOf
that searches in the other direc‐
tion, and returns the last index that matches what you’re looking for). If you only
want to search a portion of an array, you can specify an optional start index. If
indexOf
(or
lastIndexOf
) returns –
1
, it wasn’t able to find a match:
const
o
=
{
name
:
"Jerry"
};
const
arr
=
[
1
,
5
,
"a"
,
o
,
true
,
5
, [
1
,
2
],
"9"
];
arr
.
indexOf
(
5
);
// returns 1
arr
.
lastIndexOf
(
5
);
// returns 5
arr
.
indexOf
(
"a"
);
// returns 2
arr
.
lastIndexOf
(
"a"
);
// returns 2
arr
.
indexOf
({
name
:
"Jerry"
});
// returns -1
arr
.
indexOf
(
o
);
// returns 3
arr
.
indexOf
([
1
,
2
]);
// returns -1
arr
.
indexOf
(
"9"
);
// returns 7
arr
.
indexOf
(
9
);
// returns -1
arr
.
indexOf
(
"a"
,
5
);
// returns -1
arr
.
indexOf
(
5
,
5
);
// returns 5
arr
.
lastIndexOf
(
5
,
4
);
// returns 1
arr
.
lastIndexOf
(
true
,
3
);
// returns -1
Next up is
findIndex
, which is similar to
indexOf
in that it returns an index (or –
1
if
there’s no match).
findIndex
is more flexible, though, in that you can provide a func‐
tion that determines if an element is a match (
findIndex
doesn’t have an option to
start at an arbitrary index, nor is there a corresponding
findLastIndex
):
const
arr
=
[{
id
:
5
,
name
:
"Judith"
}, {
id
:
7
,
name
:
"Francis"
}];
arr
.
findIndex
(
o
=>
o
.
id
===
5
);
// returns 0
arr
.
findIndex
(
o
=>
o
.
name
===
"Francis"
);
// returns 1
arr
.
findIndex
(
o
=>
o
===
3
);
// returns -1
arr
.
findIndex
(
o
=>
o
.
id
===
17
);
// returns -1
find
and
findIndex
are great if you’re looking for the index of an element. But what
if you don’t care about the index of the element, but just want the element itself?
find
is like
findIndex
in that it allows you to specify a function to find what you’re look‐
ing for, but it returns the element itself instead of the index (or
null
if no such ele‐
ment was found):
const
arr
=
[{
id
:
5
,
name
:
"Judith"
}, {
id
:
7
,
name
:
"Francis"
}];
arr
.
find
(
o
=>
o
.
id
===
5
);
// returns object { id: 5, name: "Judith" }
arr
.
find
(
o
=>
o
.
id
===
2
);
// returns null
136 | Chapter 8: Arrays and Array Processing
