Reverse a number depending on the total number of digits

Is there a built-in function in javascript to reverse a number depending on the total number? if no how to implement it?

i.e.

number = 2;
total_number = 10;
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answer开发者_开发问答 = 9;

reverse(2, 10) // returns 9

since:

 1  == 10
[2  ==  9]
 3  ==  8
 4  ==  7
 5  ==  6
 6  ==  5
 7  ==  4
 8  ==  3
 9  ==  2
 10 ==  1

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function (num, total) {
    return total - num + 1;
}

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Shouldn't this work:

total - number + 1

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Why not:

Total_number - number + 1