What does arr.length[i].legth means

Why we use [i] what it mean plase help me

You are going to have to give some more context, we have no idea what you are referring to.

[i] is most likely used to index into an array.

Anyway, it does not look like valid code as you can not index into the length of an array, because length is just a number.

[1,2,3].length // 3

“i” is a temporary variable used to store the integer value of the current position in the range of the for loop that only has scope within its for loop. You could use any other variable name in place of “i” such as “count” or “x” or “number”.

For instance in your for loop above, during the first iteration of the loop i = 1, then i=2 for the next iteration then, i= 3 and so on.