Get difference between last two values of an RxJS Observable

Use pairwise:

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    .pairwise()
    .map(([a, b]) => b - a);

Old question, but to add info for RxJS version 5, where the API surface has changed somewhat and it took me some time to find the answer: the equivalent to pairwise would be bufferWithCount(2,1) (v4) or bufferCount(2,1) (v5).