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The Array object, as with arrays in other programming languages, enables storing a collection of multiple items under a single variable name, and has members for performing common array operations.

Array objects cannot use arbitrary strings as element indexes (as in an associative array) but must use nonnegative integers (or their respective string form). Setting or accessing via non-integers will not set or retrieve an element from the array list itself, but will set or access a variable associated with that array's object property collection. The array's object properties and list of array elements are separate, and the array's traversal and mutation operations cannot be applied to these named properties.

Array Download

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Array elements are object properties in the same way that toString is a property (to be specific, however, toString() is a method). Nevertheless, trying to access an element of an array as follows throws a syntax error because the property name is not valid:

JavaScript syntax requires properties beginning with a digit to be accessed using bracket notation instead of dot notation. It's also possible to quote the array indices (e.g., years['2'] instead of years[2]), although usually not necessary.

When setting a property on a JavaScript array when the property is a valid array index and that index is outside the current bounds of the array, the engine will update the array's length property accordingly:

Methods that have special treatment for empty slots include the following: concat(), copyWithin(), every(), filter(), flat(), flatMap(), forEach(), indexOf(), lastIndexOf(), map(), reduce(), reduceRight(), reverse(), slice(), some(), sort(), and splice(). Iteration methods such as forEach don't visit empty slots at all. Other methods, such as concat, copyWithin, etc., preserve empty slots when doing the copying, so in the end the array is still sparse.

Other methods mutate the array that the method was called on, in which case their return value differs depending on the method: sometimes a reference to the same array, sometimes the length of the new array.

The following methods create new arrays by accessing this.constructor[Symbol.species] to determine the constructor to use: concat(), filter(), flat(), flatMap(), map(), slice(), and splice() (to construct the array of removed elements that's returned).

Many array methods take a callback function as an argument. The callback function is called sequentially and at most once for each element in the array, and the return value of the callback function is used to determine the return value of the method. They all share the same signature:

The reduce() and reduceRight() methods also take a callback function and run it at most once for each element in the array, but they have slightly different signatures from typical iterative methods (for example, they don't accept thisArg).

The language avoids setting length to an unsafe integer. All built-in methods will throw a TypeError if length will be set to a number greater than 253 - 1. However, because the length property of arrays throws an error if it's set to greater than 232 - 1, the safe integer threshold is usually not reached unless the method is called on a non-array object.

The term array-like object refers to any object that doesn't throw during the length conversion process described above. In practice, such object is expected to actually have a length property and to have indexed elements in the range 0 to length - 1. (If it doesn't have all indices, it will be functionally equivalent to a sparse array.) Any integer index less than zero or greater than length - 1 is ignored when an array method operates on an array-like object.

Note: If you're not yet familiar with array basics, consider first reading JavaScript First Steps: Arrays, which explains what arrays are, and includes other examples of common array operations.

All built-in array-copy operations (spread syntax, Array.from(), Array.prototype.slice(), and Array.prototype.concat()) create shallow copies. If you instead want a deep copy of an array, you can use JSON.stringify() to convert the array to a JSON string, and then JSON.parse() to convert the string back into a new array that's completely independent from the original array.

Finally, it's important to understand that assigning an existing array to a new variable doesn't create a copy of either the array or its elements. Instead the new variable is just a reference, or alias, to the original array; that is, the original array's name and the new variable name are just two names for the exact same object (and so will always evaluate as strictly equivalent). Therefore, if you make any changes at all either to the value of the original array or to the value of the new variable, the other will change, too:

The result of a match between a RegExp and a string can create a JavaScript array that has properties and elements which provide information about the match. Such an array is returned by RegExp.prototype.exec() and String.prototype.match().

Iterative methods do not mutate the array on which it is called, but the function provided as callbackFn can. The key principle to remember is that only indexes between 0 and arrayLength - 1 are visited, where arrayLength is the length of the array at the time the array method was first called, but the element passed to the callback is the value at the time the index is visited. Therefore:

Inserting n elements at unvisited indexes that are less than the initial array length will make them be visited. The last n elements in the original array that now have index greater than the initial array length will not be visited:

Deleting n elements at unvisited indexes will make them not be visited anymore. Because the array has shrunk, the last n iterations will visit out-of-bounds indexes. If the method ignores non-existent indexes (see array methods and empty slots), the last n iterations will be skipped; otherwise, they will receive undefined:

Deleting n elements at already visited indexes does not change the fact that they were visited before they get deleted. Because the array has shrunk, the next n elements after the current index are skipped. If the method ignores non-existent indexes, the last n iterations will be skipped; otherwise, they will receive undefined: 9af72c28ce