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7.1 Creating Arrays

7.1 Creating Arrays


The easiest way to create an array is with an array literal, which is simply a comma-separated list of array elements within square brackets. For example:

var empty = []; // An array with no elements var primes = [2, 3, 5, 7, 11]; // An array with 5 numeric elements var misc = [ 1.1, true, "a", ]; // 3 elements of various types + trailing comma

The values in an array literal need not be constants; they may be arbitrary expressions:

var base = 1024; var table = [base, base+1, base+2, base+3];

Array literals can contain object literals or other array literals:

var b = [[1,{x:1, y:2}], [2, {x:3, y:4}]];

If you omit a value from an array literal, the omitted element is given the value undefined: var count = [1,,3]; // An array with 3 elements, the middle one undefined. var undefs = [,,]; // An array with 2 elements, both undefined.

Array literal syntax allows an optional trailing comma, so [,,] has only two elements, not three.

Another way to create an array is with the Array() constructor. You can invoke this constructor in three distinct ways:

• Call it with no arguments:

var a = new Array();

This method creates an empty array with no elements and is equivalent to the array literal [].

• Call it with a single numeric argument, which specifies a length:

var a = new Array(10);

This technique creates an array with the specified length. This form of the Array()constructor can be used to preallocate an array when you know in advance how many elements will be required. Note that no values are stored in the array, and the array index properties “0”, “1”, and so on are not even defined for the array.

• Explicitly specify two or more array elements or a single non-numeric element for the array:

var a = new Array(5, 4, 3, 2, 1, "testing, testing");

In this form, the constructor arguments become the elements of the new array. Using an array literal is almost always simpler than this usage of the Array() constructor.

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