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can be used to create such subarrays, where
function
is a function that returns a Boolean
value of
true
or
false
for each item in the array. The array items that result in a value of
true
get filtered or passed into a new array. The following code demonstrates how to use
the
filter()
method to create a subarray of items whose value is less than 70:
var scores = [92, 68, 83, 65, 71, 55];
var lowScores = scores.filter(getLow);
function getLow(value) {
return value < 70;
}
After running this code, the
lowScores
array would contain the values [68, 65, 55].
In another common use of arrays, you want to examine an array's contents to deter-
mine whether every array item satisfies a specified condition. The
every()
method
array
.every(
function
)
returns the value
true
if every item in an array matches the condition specified in
function
, and returns
false
otherwise. Note that the function used by the
every()
method must return a Boolean value of
true
or
false
. For example, the following code
uses the
every()
method to test whether every test score exceeds a value of 70:
var scores = [85, 92, 88, 69, 71];
var allPassing = scores.every(passTest);
function passTest(value) {
return value > 70;
}
In this example, the
allPassing
variable would be
false
because at least one array item
(69) does not exceed 70. Similarly, the
some()
method
array
.some(
function
)
returns a value of
true
or
false
if some—but not necessarily all—array items match a
condition specified in
function
.
Altogether, nine new methods for working with arrays were introduced in ECMAScript 5.
Figure 12-18 summarizes the syntax and uses of each method.
Figure 12-18
summary methods for arrays
Method
Description
every(
function
)
Tests whether the condition returned by
function
holds for all items
in the array (
true
) or at least one counter-example exists (
false
)
filter(
function
)
Creates a new array populated with the elements of the original array
that return a value of
true
from
function
forEach(
function
)
Applies
function
to each item within the array
indexOf(
value
[,
start
])
Searches the array, returning the index number of the first element
equal to
value
, starting from the optional
start
index
lastIndexOf(
value
[,
start
])
Searches backward through the array, returning the index number of
the first element equal to
value
, starting from the optional
start
index
map(
function
)
Creates a new array by passing the original array items to
function
,
which returns the mapped value of the array items
reduce(
function
)
Reduces the array by passing the original array items to
function
,
keeping only those items that return a value of
true
