array of objects
[{tips: , explain:}, {tips: , explain:}]
indexes of javascript array elements that satisfy condition
===========================
Looping over arrayswhile
let index = 0;
const array = [1,2,3,4,5,6];
while (index < array.length) {
console.log(array[index]);
index++;
}
for (classical)
const array = [1,2,3,4,5,6];
for (let index = 0; index < array.length; index++) {
console.log(array[index]);
}
forEach
const array = [1,2,3,4,5,6];
array.forEach(function(current_value, index, array) {
console.log(`At index ${index} in array ${array} the value is ${current_value}`);
});
// => undefined
map
The last construct was useful, however, it doesn’t return a new array which might be undesirable for your specific case. map solves this by applying a function over every element and then returning the new array.
const array = [1,2,3,4,5,6];
const square = x => Math.pow(x, 2);
const squares = array.map(square);
console.log(`Original array: ${array}`);
console.log(`Squared array: ${squares}`);
The full signature for map is .map(current_value, index, array).
reduce
From MDN:
The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.
const array = [1,2,3,4,5,6];
const sum = (x, y) => x + y;
const array_sum = array.reduce(sum, 0);
console.log(`The sum of array: ${array} is ${array_sum}`);
filter
Filters elements on an array based on a boolean function.
const array = [1,2,3,4,5,6];
const even = x => x % 2 === 0;
const even_array = array.filter(even);
console.log(`Even numbers in array ${array}: ${even_array}`);
every
Got an array and want to test if a given condition is met in every element?
const array = [1,2,3,4,5,6];
const under_seven = x => x < 7;
if (array.every(under_seven)) {
console.log('Every element in the array is less than 7');
} else {
console.log('At least one element in the array was bigger than 7');
}
some
Test if at least one element matches our boolean function.
const array = [1,2,3,9,5,6,4];
const over_seven = x => x > 7;
if (array.some(over_seven)) {
console.log('At least one element bigger than 7 was found');
} else {
console.log('No element bigger than 7 was found');
}
===========================
Looping Array Elements
The best way to loop through an array, is using a "for" loop:
Example
var fruits, text, fLen, i;
fruits = ["Banana", "Orange", "Apple", "Mango"];
fLen = fruits.length;
text = "<ul>";
for (i = 0; i < fLen; i++) {
text += "<li>" + fruits[i] + "</li>";
}
===========================
Adding Array Elements
The easiest way to add a new element to an array is using the push method:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.push("Lemon"); // adds a new element (Lemon) to fruits
New element can also be added to an array using the length property:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[fruits.length] = "Lemon"; // adds a new element (Lemon) to fruits
Adding elements with high indexes can create undefined "holes" in an array:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[6] = "Lemon"; // adds a new element (Lemon) to fruits
===========================
sampling-a-random-subset-from-an-array
Referrence: sampling-a-random-subset-from-an-array
function getRandomSubarray(arr, size) {
var shuffled = arr.slice(0), i = arr.length, min = i - size, temp, index;
while (i-- > min) {
index = Math.floor((i + 1) * Math.random());
temp = shuffled[index];
shuffled[index] = shuffled[i];
shuffled[i] = temp;
}
return shuffled.slice(min);
}
var x = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15];
var fiveRandomMembers = getRandomSubarray(x, 5);
===========================
Splicing an Array
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2, 0, "Lemon", "Kiwi");
The first parameter (2) defines the position where new elements should be added (spliced in).
The second parameter (0) defines how many elements should be removed.
The rest of the parameters ("Lemon" , "Kiwi") define the new elements to be added.
Using splice() to Remove Elements without leaving "holes" in the array:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(0, 1); // Removes the first element of fruits
to locate element in array
To locate element position: indexOf()
function myFunction() {
var fruits = ["Banana", "Orange", "Apple", "Mango"];
var a = fruits.indexOf("Apple");
document.getElementById("demo").innerHTML = a;
}
===========================
combine 2 arrays into a multidimensional arraycombine 2 arrays into a multidimensional array
var array1 = ["Pepsi", "Coke", "Juice", "Water"],
array2 = ["35", "17", "21", "99"],
result = [], i = -1;
while ( array1[++i] ) {
result.push( [ array1[i], array2[i] ] );
}
============
var newArray = [];
for (var i = 0; i < oneArray.length; i++) {
newArray[oneArray[i]] = data[i];
}
Array Map MethodArray Map Method
JavaScript arrays are used to store multiple values in a single variable.
An array is a special variable, which can hold more than one value at a time.
If you have a list of items (a list of car names, for example), storing the
cars in single variables could look like this:
var car1 = "Saab";
var car2 = "Volvo";
var car3 = "BMW";
However, what if you want to loop through the cars and find a specific one?
And what if you had not 3 cars, but 300?
The solution is an array!
An array can hold many values under a single name, and you can
access the values by referring to an index number.
Creating an Array
Syntax:
var array_name = [item1, item2, ...]
Example
var cars = ["Saab", "Volvo", "BMW"];
Spaces and line breaks are not important. A declaration can span multiple lines:
Example
var cars = [
"Saab", "Volvo", "BMW"
];
Putting a comma after the last element (like "BMW",) is inconsistent
across browsers.
IE 8 and earlier will fail.
Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
Example
var cars = new Array("Saab", "Volvo", "BMW");
The two examples above do exactly the same. There is no need to use new Array().
For simplicity, readability and execution speed, use the first one (the array literal method).
Access the Elements of an Array
You access an array element by referring to the index number.
This statement accesses the value of the first element in cars:
var name = cars[0];
Example
var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars[0];
Note: Array indexes start with 0.
[0] is the first element.
[1] is the second element.
Changing an Array Element
This statement changes the value of the first element in cars:
cars[0] = "Opel";
With JavaScript, the full array can be accessed by referring to the array
name:
Example
var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;
Arrays are Objects
Arrays are a special type of objects. The typeof operator in JavaScript returns "object" for
arrays.
But, JavaScript arrays are best described as arrays.
Arrays use numbers to access its "elements". In this
example, person[0]
returns John:
Array:
var person = ["John", "Doe", 46];
Objects use names to access its "members". In this example,
person.firstName
returns John:
Object:
var person = {firstName:"John", lastName:"Doe", age:46};
Array Elements Can Be Objects
JavaScript variables can be objects. Arrays are special kinds of objects.
Because of this, you can have variables of different types in the
same Array.
You can have objects in an Array. You can have functions in an Array. You can
have arrays in an Array:
myArray[0] = Date.now;
myArray[1] = myFunction;
myArray[2] = myCars;
Array Properties and Methods
The real strength of JavaScript arrays are the built-in array properties and
methods:
Examples
var x = cars.length; // The length property returns the number of elements
var y = cars.sort(); // The sort() method sorts arrays
Array methods are covered in the next chapters.
The length Property
The length property of an array returns the length of an array (the number of array
elements).
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.length; // the length of fruits is 4
The length property is always one more than the highest array index.
Accessing the First Array Element
Example
fruits = ["Banana", "Orange", "Apple", "Mango"];
var first = fruits[0];
Accessing the Last Array Element
Example
fruits = ["Banana", "Orange", "Apple", "Mango"];
var last = fruits[fruits.length - 1];
Looping Array Elements
The safest way to loop through an array, is using a for loop:
Example
var fruits, text, fLen, i;
fruits = ["Banana", "Orange", "Apple", "Mango"];
fLen = fruits.length;
text = "<ul>";
for (i = 0; i < fLen; i++) {
text += "<li>" + fruits[i] + "</li>";
}
text
+= "</ul>";
You can also use the Array.forEach() function:
Example
var fruits, text;
fruits = ["Banana", "Orange", "Apple", "Mango"];
text = "<ul>";
fruits.forEach(myFunction);
text += "</ul>";
function
myFunction(value) {
text += "<li>" + value + "</li>";
}
Adding Array Elements
The easiest way to add a new element to an array is using the push() method:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.push("Lemon"); // adds a new element (Lemon) to fruitsTry it Yourself »
New element can also be added to an array using the length property:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[fruits.length] = "Lemon"; // adds a new element (Lemon) to fruitsTry it Yourself »WARNING !
Adding elements with high indexes can create undefined "holes" in an array:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[6] = "Lemon"; // adds a new element (Lemon) to fruitsTry it Yourself »
Associative Arrays
Many programming languages support arrays with named indexes.
Arrays with named indexes are called associative
arrays (or hashes).
JavaScript does not support arrays with named indexes.
In JavaScript, arrays always use numbered indexes.
Example
var person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
var x = person.length;
// person.length will return 3
var y = person[0];
// person[0] will return "John"Try it Yourself »WARNING !!
If you use named indexes, JavaScript will redefine the array to a standard object.
After that, some array methods and properties will produce incorrect
results.
Example:
var person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
var x = person.length; // person.length will
return 0
var y = person[0];
// person[0] will return undefinedTry it Yourself »
The Difference Between Arrays and Objects
In JavaScript, arrays use numbered indexes.
In JavaScript, objects use named indexes.
Arrays are a special kind of objects, with numbered indexes.
When to Use Arrays. When to use Objects.
JavaScript does not support associative arrays.
You should use objects when you want the element names to be
strings (text).
You should use arrays when you want the element names to be
numbers.
Avoid new Array()
There is no need to use the JavaScript's built-in array constructor new Array().
Use [] instead.
These two different statements both create a new empty array named points:
var points = new Array(); // Bad
var points = []; // Good
These two different statements both create a new array containing 6 numbers:
var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good
Try it Yourself »
The new keyword only complicates the code. It can also
produce some unexpected results:
var points = new Array(40, 100); // Creates an array with two elements (40 and 100)
What if I remove one of the elements?
var points = new Array(40); // Creates an array with 40 undefined elements !!!!!Try it Yourself »
How to Recognize an Array
A common question is: How do I know if a variable is an array?
The problem is that the JavaScript operator typeof returns
"object":
var fruits = ["Banana", "Orange", "Apple", "Mango"];
typeof fruits; // returns objectTry it Yourself »
The typeof operator returns object because a JavaScript array is an
object.
Solution 1:
To solve this problem ECMAScript 5 defines a new method Array.isArray():
Array.isArray(fruits); // returns true
The problem with this solution is that ECMAScript 5 is not supported
in older browsers.
Solution 2:
To solve this problem you can create your own isArray() function:
function isArray(x) {
return x.constructor.toString().indexOf("Array") > -1;
}Try it Yourself »
The function above always returns true if the argument is an array.
Or more precisely: it returns true if the object prototype contains the word
"Array".
Solution 3:
The instanceof operator returns true if an object is created
by a given constructor:
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits instanceof Array; // returns true
Test Yourself With Exercises
Exercise:
Get the value "Volvo" from the cars array.
var cars = ["Saab", "Volvo", "BMW"];
var x = ;
JavaScript Array Methods
Converting Arrays to Strings
The JavaScript method toString() converts an array to a
string of (comma separated) array values.
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.toString();
Result:
Banana,Orange,Apple,Mango
Try it Yourself »
The join() method also joins all array elements into a string.
It behaves just like toString(), but in addition you can specify the separator:
When you work with arrays, it is easy to remove elements and add
new elements.
This is what popping and pushing is:
Popping items out of an array, or pushing
items into an array.
Popping
The pop() method removes the last element from an array:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.pop();
// Removes the last element ("Mango") from fruitsTry it Yourself »
The pop() method returns the value that was "popped out":
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
var x =
fruits.pop();
// the value of x is "Mango"Try it Yourself »
Pushing
The push() method adds a new element to an array (at the end):
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.push("Kiwi"); // Adds a new element ("Kiwi") to fruitsTry it Yourself »
The push() method returns the new array length:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
var x =
fruits.push("Kiwi"); // the value of x is 5Try it Yourself »
Shifting Elements
Shifting is equivalent to popping, working on the first element instead of
the last.
The shift() method removes the first array element and "shifts" all
other elements to a lower index.
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.shift();
// Removes the first element "Banana" from fruits
Try it Yourself »
The shift() method returns the string that was "shifted out":
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
var x =
fruits.shift(); // the value of x is "Banana"
The unshift() method adds a new element to an array (at the beginning), and "unshifts"
older elements:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.unshift("Lemon");
// Adds a new element "Lemon" to fruits
Try it Yourself »
The unshift() method returns the new array length.
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.unshift("Lemon");
// Returns 5
Try it Yourself »
Changing Elements
Array elements are accessed using their index number:
Array indexes start with 0.
[0] is the first array element, [1] is the second, [2] is the third ...
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[0] = "Kiwi";
// Changes the first element of fruits to "Kiwi"Try it Yourself »
The length property provides an easy way to append a new element to an array:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[fruits.length] = "Kiwi";
// Appends "Kiwi" to fruitsTry it Yourself »
Deleting Elements
Since JavaScript arrays are objects, elements can be deleted by using the
JavaScript operator delete:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
delete fruits[0];
// Changes the first element in fruits to undefinedTry it Yourself »
Using delete may leave undefined holes in the
array. Use pop() or shift() instead.
Splicing an Array
The splice() method can be used to add new items to an array:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2, 0, "Lemon", "Kiwi");
Try it Yourself »
The first parameter (2) defines the position where new elements should be
added (spliced in).
The second parameter (0) defines how many elements should be
removed.
The rest of the parameters ("Lemon" , "Kiwi") define the new elements to be
added.
The splice() method returns an array with the deleted items:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2, 2, "Lemon", "Kiwi");
Try it Yourself »
Using splice() to Remove Elements
With clever parameter setting, you can use splice() to remove elements without leaving
"holes" in the array:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(0, 1);
// Removes the first element of fruits
Try it Yourself »
The first parameter (0) defines the position where new elements should be
added (spliced in).
The second parameter (1) defines how many elements should be
removed.
The rest of the parameters are omitted. No new elements will be added.
Merging (Concatenating) Arrays
The concat() method creates a new array by merging (concatenating)
existing arrays:
Example (Merging Two Arrays)
var myGirls = ["Cecilie", "Lone"];
var myBoys = ["Emil", "Tobias", "Linus"];
var myChildren = myGirls.concat(myBoys);
// Concatenates (joins) myGirls and myBoys
Try it Yourself »
The concat() method does not change the existing arrays. It always returns a new array.
The concat() method can take any number of array arguments:
Example (Merging Three Arrays)
var arr1 = ["Cecilie", "Lone"];
var arr2 = ["Emil", "Tobias", "Linus"];
var arr3 = ["Robin", "Morgan"];
var myChildren = arr1.concat(arr2, arr3); // Concatenates arr1 with arr2 and arr3
Try it Yourself »
The concat() method can also take values as arguments:
Example (Merging an Array with Values)
var arr1 = ["Cecilie", "Lone"];
var myChildren = arr1.concat(["Emil", "Tobias", "Linus"]);
Slicing an Array
The slice() method slices out a piece of an array into a new
array.
This example slices out a part of an array starting from array element 1
("Orange"):
Example
var fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = fruits.slice(1);
Try it Yourself »
The slice() method creates a new array. It does not remove any elements from
the source array.
This example slices out a part of an array starting from array element 3
("Apple"):
Example
var fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = fruits.slice(3);
Try it Yourself »
The slice() method can take two arguments like slice(1, 3).
The method then selects elements from the start argument, and up to (but not
including) the end argument.
Example
var fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = fruits.slice(1, 3);
Try it Yourself »
If the end argument is omitted, like in the first examples, the slice()
method slices out the rest of the array.
Example
var fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = fruits.slice(2);
Try it Yourself »
Automatic toString()
JavaScript automatically converts an array to a comma separated string when a
primitive value is expected.
This is always the case when you try to output an array.
These two examples will produce the same result:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.toString();
Try it Yourself »
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits;
Try it Yourself »
All JavaScript objects have a toString() method.
Finding Max and Min Values in an Array
There are no built-in functions for finding the highest
or lowest value in a JavaScript array.
You will learn how you solve this problem in the next
chapter of this tutorial.
Sorting Arrays
Sorting arrays are covered in the next chapter of this tutorial.
Complete Array Reference
For a complete reference, go to our Complete
JavaScript Array Reference.
The reference contains descriptions and examples of all Array
properties and methods.