7.3 Sparse Arrays
A sparse array is one in which the elements do not have contiguous indexes starting at
0. Normally, the length property of an array specifies the number of elements in the array. If the array is sparse, the value of the length property is greater than the number of elements. Sparse arrays can be created with the Array() constructor or simply by assigning to an array index larger than the current array length.
a = new Array(5); // No elements, but a.length is 5. a = ; // Create an array with no elements and length = 0. a = 0; // Assignment adds one element but sets length to 1001.
We’ll see later that you can also make an array sparse with the delete operator.
Arrays that are sufficiently sparse are typically implemented in a slower, more memory-efficient way than dense arrays are, and looking up elements in such an array will take about as much time as regular object property lookup.
Note that when you omit value in an array literal, you are not creating a sparse array. The omitted element exists in the array and has the value undefined. This is subtly different than array elements that do not exist at all. You can detect the difference between these two cases with the in operator:
var a1 = [,,,]; // This array is [undefined, undefined, undefined] var a2 = new Array(3); // This array has no values at all 0 in a1 // => true: a1 has an element with index 0 0 in a2 // => false: a2 has no element with index 0
The difference between a1and a2is also apparent when you use a for/inloop. See §7.6 .欢迎转载,转载请注明来自一手册:http://yishouce.com/book/1/27652.html