JavaScript Array Iteration
Array iteration methods operate on every array item.
Array.forEach()
The forEach() method calls a function (a callback function) once for each array element.
Example
const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);
function myFunction(value, index, array) {
txt += value + "<br>";
}
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Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
The example above uses only the value parameter. The example can be rewritten to:
Example
const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);
function myFunction(value) {
txt += value + "<br>";
}
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Array.map()
The map() method creates a new array by performing a function on each array element.
The map() method does not execute the function for array
elements without values.
The map() method does not change the original array.
This example multiplies each array value by 2:
Example
const numbers1 = [45, 4, 9, 16, 25];
const numbers2 = numbers1.map(myFunction);
function myFunction(value, index, array) {
return value * 2;
}
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Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
When a callback function uses only the value parameter, the index and array parameters can be omitted:
Example
const numbers1 = [45, 4, 9, 16, 25];
const numbers2 = numbers1.map(myFunction);
function myFunction(value) {
return value * 2;
}
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Array.filter()
The filter() method creates a new array with array elements that passes a test.
This example creates a new array from elements with a value larger than 18:
Example
const numbers = [45, 4, 9, 16, 25];
const over18 = numbers.filter(myFunction);
function myFunction(value, index, array) {
return value > 18;
}
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Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
In the example above, the callback function does not use the index and array parameters, so they can be omitted:
Example
const numbers = [45, 4, 9, 16, 25];
const over18 =
numbers.filter(myFunction);
function myFunction(value) {
return value > 18;
}
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Array.reduce()
The reduce() method runs a function on each array element to produce (reduce it to) a single value.
The reduce() method works from left-to-right in the array. See also reduceRight().
The reduce() method does not reduce the original array.
This example finds the sum of all numbers in an array:
Example
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);
function myFunction(total, value, index, array) {
return total + value;
}
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Note that the function takes 4 arguments:
- The total (the initial value / previously returned value)
- The item value
- The item index
- The array itself
The example above does not use the index and array parameters. It can be rewritten to:
Example
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);
function myFunction(total, value) {
return total + value;
}
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The reduce() method can accept an initial value:
Example
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction,
100);
function myFunction(total, value) {
return total + value;
}
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Array.reduceRight()
The reduceRight() method runs a function on each array element to produce (reduce it to) a single value.
The reduceRight() works from right-to-left in the array. See also reduce().
The reduceRight() method does not reduce the original array.
This example finds the sum of all numbers in an array:
Example
const numbers = [45, 4, 9, 16, 25];
let sum = numbers1.reduceRight(myFunction);
function myFunction(total, value, index, array) {
return total + value;
}
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Note that the function takes 4 arguments:
- The total (the initial value / previously returned value)
- The item value
- The item index
- The array itself
The example above does not use the index and array parameters. It can be rewritten to:
Example
const numbers = [45, 4, 9, 16, 25];
let sum = numbers1.reduceRight(myFunction);
function myFunction(total, value) {
return total + value;
}
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Array.every()
The every() method check if all array values pass a test.
This example check if all array values are larger than 18:
Example
const numbers = [45, 4, 9, 16, 25];
let allOver18 =
numbers.every(myFunction);
function myFunction(value, index, array) {
return
value > 18;
}
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Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
When a callback function uses the first parameter only (value), the other parameters can be omitted:
Example
const numbers = [45, 4, 9, 16, 25];
let allOver18 =
numbers.every(myFunction);
function myFunction(value) {
return
value > 18;
}
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Array.some()
The some() method check if some array values pass a test.
This example check if some array values are larger than 18:
Example
const numbers = [45, 4, 9, 16, 25];
let someOver18 = numbers.some(myFunction);
function myFunction(value, index, array) {
return
value > 18;
}
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Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
Array.indexOf()
The indexOf() method searches an array for an element value and returns its position.
Note: The first item has position 0, the second item has position 1, and so on.
Example
Search an array for the item "Apple":
const fruits = ["Apple", "Orange", "Apple", "Mango"];
let position = fruits.indexOf("Apple") + 1;
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Syntax
array.indexOf(item, start)
| item | Required. The item to search for. |
| start | Optional. Where to start the search. Negative values will start at the given position counting from the end, and search to the end. |
Array.indexOf() returns -1 if the item is not found.
If the item is present more than once, it returns the position of the first occurrence.
Array.lastIndexOf()
Array.lastIndexOf() is the same as Array.indexOf(), but
returns the position of the last occurrence of the specified element.
Example
Search an array for the item "Apple":
const fruits = ["Apple", "Orange", "Apple", "Mango"];
let position = fruits.lastIndexOf("Apple") + 1;
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Syntax
array.lastIndexOf(item, start)
| item | Required. The item to search for |
| start | Optional. Where to start the search. Negative values will start at the given position counting from the end, and search to the beginning |
Array.includes()
ECMAScript 2016 introduced Array.includes() to arrays.
This allows us to check if an element is present in an array (including NaN, unlike indexOf).
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.includes("Mango"); // is true
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Syntax
array.includes(search-item)
Array.includes() allows to check for NaN values. Unlike Array.indexOf().
Array.includes() is not supported in Internet Explorer and Edge 12/13.
The first browser versions with full support are:
| Chrome 47 | Edge 14 | Firefox 43 | Safari 9 | Opera 34 |
| Des 2015 | Aug 2016 | Des 2015 | Oct 2015 | Des 2015 |
Array.find()
The find() method returns the value of the first array element that passes a
test function.
This example finds (returns the value of) the first element that is larger than 18:
Example
const numbers = [4, 9, 16, 25, 29];
let first =
numbers.find(myFunction);
function myFunction(value, index, array) {
return
value > 18;
}
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Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
Array.find() is not supported in older browsers. The first browser versions
with full support are:
| Chrome 45 | Edge 12 | Firefox 25 | Safari 8 | Opera 32 |
| Sep 2015 | Aug 2015 | Oct 2013 | Oct 2014 | Sep 2015 |
Array.findIndex()
The findIndex() method returns the index of the first array element that
passes a test function.
This example finds the index of the first element that is larger than 18:
Example
const numbers = [4, 9, 16, 25, 29];
let first =
numbers.findIndex(myFunction);
function myFunction(value, index, array) {
return
value > 18;
}
Try it Yourself »
Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
Array.findIndex() is not supported in older browsers. The first browser
versions with full support are:
| Chrome 45 | Edge 12 | Firefox 25 | Safari 8 | Opera 32 |
| Sep 2015 | Aug 2015 | Oct 2013 | Oct 2014 | Sep 2015 |
