Binary search over a sorted list

Sorted data turns up constantly on the client: a timeline of messages by timestamp, a price ladder, a list of sorted option values in a combobox. Scanning it linearly wastes the fact that it is sorted.

Implement binarySearch(sorted, target). sorted is an array of numbers in ascending order with no duplicate values, and target is a number. Return the index of target in the array, or -1 when the array does not contain it. An empty array always returns -1.

What it has to do

• Return the index of the target when it is present.
• Return -1 when the target is absent.
• Return -1 for an empty array.
• Halve the search range each step rather than scanning every element.

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