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Possible Duplicate:
Are Javascript arrays sparse?
Is the following “safe” in JavaScript? (as in, can be expected to work reliably on all JavaScript engines)
a = [];
a[100] = "hello";
a[100] == "hello"; // should be true
Yes. Arrays in JavaScript are sparse and your code is expected to work in all JavaScript implementations.
You can get into requirements in the section 15.4 of the specification(PDF).
Short summary: array is special object that have length property adjusted when one adds elements at properties with numeric names (like `a[123]=”test”). Other methods like join take length into account duuring operations.