JavaScript Array Iteration
Array iteration methods operate on every array item.
JavaScript Array forEach()
这 forEach() method calls a function (a callback function) once for each array element.
例子
const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);
function myFunction(value, index, array) {
txt += value + "<br>";
}
亲自尝试 »
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:
例子
const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);
function myFunction(value) {
txt += value + "<br>";
}
亲自尝试 »
JavaScript Array map()
这 map() method creates a new array by performing a function on each array element.
这 map() method does not execute the function for array elements without values.
这 map() method does not change the original array.
This example multiplies each array value by 2:
例子
const numbers1 = [45, 4, 9, 16, 25];
const numbers2 = numbers1.map(myFunction);
function myFunction(value, index, array) {
return value * 2;
}
亲自尝试 »
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:
例子
const numbers1 = [45, 4, 9, 16, 25];
const numbers2 = numbers1.map(myFunction);
function myFunction(value) {
return value * 2;
}
亲自尝试 »
JavaScript Array filter()
这 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:
例子
const numbers = [45, 4, 9, 16, 25];
const over18 = numbers.filter(myFunction);
function myFunction(value, index, array) {
return value > 18;
}
亲自尝试 »
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:
例子
const numbers = [45, 4, 9, 16, 25];
const over18 = numbers.filter(myFunction);
function myFunction(value) {
return value > 18;
}
亲自尝试 »
JavaScript Array reduce()
这 reduce() method runs a function on each array element to produce (reduce it to) a single value.
这 reduce() method works from left-to-right in the array. See also reduceRight().
这 reduce() method does not reduce the original array.
This example finds the sum of all numbers in an array:
例子
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);
function myFunction(total, value, index, array) {
return total + value;
}
亲自尝试 »
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:
例子
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);
function myFunction(total, value) {
return total + value;
}
亲自尝试 »
这 reduce() method can accept an initial value:
例子
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction, 100);
function myFunction(total, value) {
return total + value;
}
亲自尝试 »
JavaScript Array reduceRight()
这 reduceRight() method runs a function on each array element to produce (reduce it to) a single value.
这 reduceRight() works from right-to-left in the array. See also reduce().
这 reduceRight() method does not reduce the original array.
This example finds the sum of all numbers in an array:
例子
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduceRight(myFunction);
function myFunction(total, value, index, array) {
return total + value;
}
亲自尝试 »
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:
例子
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduceRight(myFunction);
function myFunction(total, value) {
return total + value;
}
亲自尝试 »
JavaScript Array every()
这 every() method check if all array values pass a test.
This example check if all array values are larger than 18:
例子
const numbers = [45, 4, 9, 16, 25];
let allOver18 = numbers.every(myFunction);
function myFunction(value, index, array) {
return value > 18;
}
亲自尝试 »
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:
例子
const numbers = [45, 4, 9, 16, 25];
let allOver18 = numbers.every(myFunction);
function myFunction(value) {
return value > 18;
}
亲自尝试 »
JavaScript Array some()
这 some() method check if some array values pass a test.
This example check if some array values are larger than 18:
例子
const numbers = [45, 4, 9, 16, 25];
let someOver18 = numbers.some(myFunction);
function myFunction(value, index, array) {
return value > 18;
}
亲自尝试 »
Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
JavaScript Array indexOf()
这 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.
例子
Search an array for the item "Apple":
const fruits = ["Apple", "Orange", "Apple", "Mango"];
let position = fruits.indexOf("Apple") + 1;
亲自尝试 »
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.
JavaScript Array lastIndexOf()
Array.lastIndexOf() is the same as Array.indexOf(), but returns the position of the last occurrence of the specified element.
例子
Search an array for the item "Apple":
const fruits = ["Apple", "Orange", "Apple", "Mango"];
let position = fruits.lastIndexOf("Apple") + 1;
亲自尝试 »
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 |
JavaScript Array find()
这 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:
例子
const numbers = [4, 9, 16, 25, 29];
let first = numbers.find(myFunction);
function myFunction(value, index, array) {
return value > 18;
}
亲自尝试 »
Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
Browser Support
find() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:
| Chrome | Edge | Firefox | Safari | Opera |
| Yes | Yes | Yes | Yes | Yes |
find() is not supported in Internet Explorer.
JavaScript Array findIndex()
这 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:
例子
const numbers = [4, 9, 16, 25, 29];
let first = numbers.findIndex(myFunction);
function myFunction(value, index, array) {
return value > 18;
}
亲自尝试 »
Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
Browser Support
findIndex() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:
| Chrome | Edge | Firefox | Safari | Opera |
| Yes | Yes | Yes | Yes | Yes |
findIndex() is not supported in Internet Explorer.
JavaScript Array.from()
这 Array.from() method returns an Array object from any object with a length property or any iterable object.
Browser Support
from() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:
| Chrome | Edge | Firefox | Safari | Opera |
| Yes | Yes | Yes | Yes | Yes |
from() is not supported in Internet Explorer.
JavaScript Array Keys()
这 Array.keys() method returns an Array Iterator object with the keys of an array.
例子
Create an Array Iterator object, containing the keys of the array:
const fruits = ["Banana", "Orange", "Apple", "Mango"];
const keys = fruits.keys();
for (let x of keys) {
text += x + "<br>";
}
亲自尝试 »
Browser Support
keys() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:
| Chrome | Edge | Firefox | Safari | Opera |
| Yes | Yes | Yes | Yes | Yes |
keys() is not supported in Internet Explorer.
Array entries()
例子
Create an Array Iterator, and then iterate over the key/value pairs:
const fruits = ["Banana", "Orange", "Apple", "Mango"];
const f = fruits.entries();
for (let x of f) {
document.getElementById("demo").innerHTML += x;
}
亲自尝试 »
这 entries() method returns an Array Iterator object with key/value pairs:
[0, "Banana"]
[1, "Orange"]
[2, "Apple"]
[3, "Mango"]
这 entries() method does not change the original array.
Browser Support
entries() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:
| Chrome | Edge | Firefox | Safari | Opera |
| Yes | Yes | Yes | Yes | Yes |
entries() is not supported in Internet Explorer.
JavaScript 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).
例子
const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.includes("Mango"); // is true
亲自尝试 »
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:
Browser Support
includes() is an ECMAScript 2016 feature.
It is supported in all modern browsers:
| Chrome | Edge | Firefox | Safari | Opera |
| Yes | Yes | Yes | Yes | Yes |
includes() is not supported in Internet Explorer.
Complete Array Reference
For a complete Array reference, go to our:
Complete JavaScript Array Reference.
The reference contains descriptions and examples of all Array properties and methods.
