I have followed these links
How [1,2] + [4,5,6][1] = 1,25 in JavaScript
Why does [5,6,8,7][1,2] = 8 in JavaScript?
but not able to understand why the following code is undefined. My understanding is [1,2,3] will give 3, so from [1,2,3,4,5,6,7][3] will be 4 and hence [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] will be [1, 2, 3, 4, 5, 6, 7, 8, 9, 10][4] that is 5. But it logs undefined.
console.log([1, 2, 3, 4, 5, 6, 7, 8, 9, 10][1, 2, 3, 4, 5, 6, 7][1, 2, 3])
You are on the right track. The second and third group of [...] are a property access with bracket notation, and the 1, 2, 3 are treated as comma operators, evaluating to the last entry in the list. So your code is basically the same as:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10][7][3]
Now the seventh number in the array is 8:
8[3]
and that does not have a "3" property.
Breaking it down into the two steps actually executed may help. First you get an 8, a number, then you try to index it – undefined ahoy!
> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10][1, 2, 3, 4, 5, 6, 7]
8
> 8[1,2,3]
undefined
You define 1D array a=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] but you treat it as 2D array during value reading a[...][...] (the a[...] returns array element - which is number - not array). This is the mistake in idea behind your code.
a=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
console.log( a[0][0] );