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Set array elements

Just as you can get array elements by an index, you can set them by this index. It does not matter whether the array was previously filled or empty.
var languages = [];
languages[0] = 'C';
languages[1] = 'C++';
languages[2] = 'Java';
Here an empty array is filled successively with the listed values.

Exercise

Write a function setFirstElement that takes an array and an arbitrary variable. The variable should be inserted as the first element in the array. The array should be returned. setFirstElement([1, 2], 3) should return [3, 2].