Arrays
Arrays are zero-indexed, ordered lists of values. They are a handy way to store a set of related items of the same type (such as strings), though in reality, an array can include multiple types of items, including other arrays.
To create an array, either use the object constructor or the literal declaration, by assigning the variable a list of values after the declaration.
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The literal declaration is generally preferred. See the Google Coding Guidelines for more information.
If the values are unknown, it is also possible to declare an empty Array, and add elements either through functions or through accessing by index:
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'push' is a function that adds an element on the end of the array and expands the array respectively. You also can directly add items by index. Missing indices will be filled with 'undefined'.
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If the size of the array is unknown, 'push' is far more safe. You can both access and assign values to array items with the index.
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Array Methods and Properties
.length
The .length
property is used to determine the amount of items in an array.
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You will need the .length
property for looping through an array:
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Except when using for
/in
loops:
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.concat
Concatenate two or more arrays with .concat
:
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.join
.join
creates a string representation of the array. Its parameter is a string that works as a separator between elements (default separator is a comma):
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.pop
.pop
removes the last element of an array. It is the opposite method of .push
:
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.reverse
As the name suggests, the elements of the array are in reverse order after calling this method:
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.shift
Removes the first element of an array. With .push
and .shift
, you can recreate the method of a queue):
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.slice
Extracts a part of the array and returns that part in a new array. This method takes one parameter, which is the starting index:
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.splice
Removes a certain amount of elements and adds new ones at the given index. It takes at least 3 parameters:
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- Index - The starting index.
- Length - The number of elements to remove.
- Values - The values to be inserted at the Index position.
For example:
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.sort
Sorts an array. It takes one parameter, which is a comparing function. If this function is not given, the array is sorted ascending:
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The return value of descending (for this example) is important. If the return value is less than zero, the index of a is before b, and if it is greater than zero it's vice-versa. If the return value is zero, the elements index is equal.
.unshift
Inserts an element at the first position of the array:
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.forEach
In modern browsers it is possible to traverse through arrays with a .forEach
method, where you pass a function that is called for each element in the array.
The function takes up to three arguments: Element - The element itself. Index - The index of this element in the array. Array* - The array itself.
All of these are optional, but you will need at least the 'element' parameter in most cases.
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