How to create an associative array in JavaScript. JQuery - Looping through Array, Object and Elements A Note on Runtime Objects

• I. Iterating over real arrays
• forEach method and related methods
• for loop
• Proper Use for...in loop
• for...of loop (implicit use of iterator)
• Explicit use of iterator
• Using methods to iterate over real arrays
• Convert to a real array
• A note on runtime objects

I. Enumeration of real arrays On this moment There are three ways to iterate over the elements of a real array:

method Array.prototype.forEach ;

classic for loop

a “correctly” constructed for...in loop.

In addition, soon, with the advent of the new ECMAScript 6 (ES 6) standard, two more methods are expected:

for...of loop (implicit use of iterator);

explicit use of iterator.

1. The forEach Method and Related Methods If your project is designed to support the features of the ECMAScript 5 (ES5) standard, you can use one of its innovations - the forEach method.

Usage example:
var a = ["a", "b", "c"]; a.forEach(function(entry) ( console.log(entry); ));
IN general case using forEach requires connecting the es5-shim emulation library for browsers that do not have native support for this method. These include IE 8 and earlier, which are still in use in some places.

The advantage of forEach is that there is no need to declare local variables to store the index and value of the current array element, since they are automatically passed to the callback function as arguments.

If you're worried about the possible cost of calling a callback on each element, don't worry and read this.

ForEach is designed to iterate over all elements of an array, but in addition to it, ES5 offers several more useful methods for iterating through all or some elements plus performing some actions on them:

• every - returns true if for each element of the array the callback returns a value that can be converted to true .
• some - returns true if for at least one element of the array the callback returns a value that can be converted to true.
• filter - creates a new array that includes those elements of the original array for which the callback returns true .
• map - creates a new array consisting of the values ​​returned by the callback.
• reduce - reduces an array to a single value, applying a callback to each element of the array in turn, starting with the first (can be useful for calculating the sum of array elements and other summary functions).
• reduceRight - works similar to reduce, but iterates through elements in reverse order.

2. For loop Good old for rules:

Var a = ["a", "b", "c"]; var index; for (index = 0; index< a.length; ++index) { console.log(a); }
If the length of the array is constant throughout the loop, and the loop itself belongs to a performance-critical section of the code (which is unlikely), then you can use the “more optimal” version for with storing the length of the array:

Var a = ["a", "b", "c"]; var index, len; for (index = 0, len = a.length; index< len; ++index) { console.log(a); }
In theory, this code should run a little faster than the previous one.

If the order of the elements is not important, then you can go even further in terms of optimization and get rid of the variable for storing the length of the array, changing the order of the search to the reverse:

Var a = ["a", "b", "c"]; var index; for (index = a.length - 1; index >= 0; --index) ( console.log(a); )
However, in modern JavaScript engines such optimization games usually mean nothing.

3. Proper Use of a for...in Loop If you are advised to use a for...in loop, remember that iterating over arrays is not what it is intended for. Contrary to a common misconception, the for...in loop does not iterate over array indices, but rather through enumerable properties of an object.

However, in some cases, such as iterating over sparse arrays, for...in can be useful, as long as you take precautions, as shown in the example below:

// a - sparse array var a = ; a = "a"; a = "b"; a = "c"; for (var key in a) ( if (a.hasOwnProperty(key) && /^0$|^\d*$/.test(key) && key

• HTML
• JavaScript

$("ul#myList").children().each(function())( console.log($(this).text()); )); // Result: // HTML // CSS // JavaScript

Let's look at a way to determine the last index (item) in jQuery's each method.

// select elements var myList = $("ul li"); // determine the number of elements in the selection var total = myList.length; // iterate through the selected elements myList.each(function(index) ( if (index === total - 1) ( // this is the last element in the selection ) ));

Last update: 03/26/2018

The Array object represents an array and provides a number of properties and methods with which we can manipulate the array.

Initializing an Array

You can create an empty array using square brackets or the Array constructor:

Var users = new Array(); var people = ; console.log(users); // Array console.log(people); //Array

You can immediately initialize an array with a certain number of elements:

Var users = new Array("Tom", "Bill", "Alice"); var people = ["Sam", "John", "Kate"]; console.log(users); // ["Tom", "Bill", "Alice"] console.log(people); // ["Sam", "John", "Kate"]

You can define an array and add new elements to it as you go:

Var users = new Array(); users = "Tom"; users = "Kate"; console.log(users); // "Tom" console.log(users); // undefined

It does not matter that by default the array is created with zero length. Using indexes, we can substitute one or another element in an array at a specific index.

length

To find out the length of an array, use the length property:

Var fruit = new Array(); fruit = "apples"; fruit = "pears"; fruit = "plums"; document.write("In the array fruit " + fruit.length + " element:
"); for(var i=0; i< fruit.length; i++) document.write(fruit[i] + "
");

In fact, the length of the array will be the index of the last element plus one. For example:

Var users = new Array(); // there are 0 elements in the array users = "Tom"; users = "Kate"; users = "Sam"; for(var i=0; i 0) ( result = true; ) return result; ); var passed = numbers.every(condition); document.write(passed); // false

The every() method is passed a function representing the condition as a parameter. This function takes three parameters:

Function condition(value, index, array) ( )

The value parameter represents the current array element being iterated, the index parameter represents the index of that element, and the array parameter passes a reference to the array.

In this function we can check the passed element value for compliance with some condition. For example, in this example we check each element of the array to see if it is greater than zero. If it is greater, then we return the value true , that is, the element meets the condition. If less, then return false - the element does not meet the condition.

As a result, when the numbers.every(condition) method is called, it iterates through all the elements of the numbers array and passes them one by one to the condition function. If this function returns true for all elements, then the every() method returns true. If at least one element does not match the condition, then the every() method returns false .

some()

The some() method is similar to the every() method, only it checks whether at least one element matches a condition. And in this case, the some() method returns true . If there are no elements matching the condition in the array, false is returned:

Var numbers = [ 1, -12, 8, -4, 25, 42 ]; function condition(value, index, array) ( var result = false; if (value === 8) ( result = true; ) return result; ); var passed = numbers.some(condition); // true

filter()

The filter() method, like some() and every() , accepts a condition function. But at the same time it returns an array of those elements that meet this condition:

Var numbers = [ 1, -12, 8, -4, 25, 42 ]; function condition(value, index, array) ( var result = false; if (value > 0) ( result = true; ) return result; ); var filteredNumbers = numbers.filter(condition); for(var i=0; i< filteredNumbers.length; i++) document.write(filteredNumbers[i] + "
");

Browser output:

1 8 25 42

forEach() and map()

The forEach() and map() methods iterate over elements and perform certain operations on them. For example, to calculate the squares of numbers in an array, you can use the following code:

Var numbers = [1, 2, 3, 4, 5, 6]; for(var i = 0; i