#JavaScript (ES6), 70 67 64 bytes

a=>(z=[...Array(a)].map((b,c)=>c+1)).join``- -z.reverse().join``

Fixed to meet requirement, as old code was made under misunderstanding of the input.

JavaScript (ES6), 70 67 64 bytes

a=>(z=[...Array(a)].map((b,c)=>c+1)).join``- -z.reverse().join``

Fixed to meet requirement, as old code was made under misunderstanding of the input.

#JavaScript (ES6), 67 bytes

a=>[...z=[...Array(a)].map((b,c)=>c+1)].reverse().join``- -z.join``

Fixed to meet requirement, as old code was made under misunderstanding of the input.

#JavaScript (ES6), 70 67 64 bytes

a=>(z=[...Array(a)].map((b,c)=>c+1)).join``- -z.reverse().join``

Fixed to meet requirement, as old code was made under misunderstanding of the input.

#JavaScript (ES6), 31 bytes

a=>+[...a].reverse().join``+a*1

Takes input as a numeric string.

The function takes in a string and splits it into an array (using [...a]) in order to take advantage of the reverse function, then join to put it back together. The first + is to coerce the reversed string into a number again. Now we add it to the original number with +a*1, where the *1 is likewise used to coerce it into a number.

#JavaScript (ES6), 67 bytes

a=>[...z=[...Array(a)].map((b,c)=>c+1)].reverse().join``- -z.join``

Fixed to meet requirement, as old code was made under misunderstanding of the input.