Swap two numbers without a third variable

Write a function swapPair(pair) that swaps two numbers without a third variable.

A function cannot change the variables of the code that called it, so the two numbers come in an array: pair is [a, b].

• Change pair itself so that it becomes [b, a], then return that same array, not a new one.
• a and b are integers from -1000000000000 to 1000000000000. They can be negative, zero or equal.
• No third variable: inside the function do not write let, const or var, do not add a second parameter, and do not write another function in it (its parameters would be variables too).
• Keep values only in the two items of pair: do not add items or properties to the array or to the function, and do not create a global variable (temp = pair[0] without let creates one).