What internal method is calling in javascript when I get array element value by index?
javascript
Solution
The ES5 spec tells us the following of array initialisers:
Elided array elements are not defined.
Note that they are not defined. That's different from having the value `undefined`. As you've already noticed, elided elements do contribute to the length of the array:
...the missing array element contributes to the length of the Array and increases the index of subsequent elements.
When you invoke `indexOf` on an array this is one of the steps that happens:
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).
In that, `k` is a number corresponding to an array index and `O` is the array itself. Since elided elements were not defined the array does not have a property for the corresponding index.
Problem
I have some `wtfjs` code: ``` var a = [,]; alert(a.indexOf(a[0])); ``` `a.indexOf(a[0])` returns `-1`. The main point in this example is difference between `uninitialized` and `undefined` values: `a` contains one not initialized element. `a[0]` return `undefined`. `a` don't contains the `undefined` value. So `a.indexOf(a[0]) === -1` is `true`. But where I can find the explanation why `a[0]` return `undefined`? What internal method is calling? P.S. `Undefined` is the javascript primitive type. `Uninitialized` means the value that don't have any javascript type, but there is no such primitive type in javascript.