Projects each source value to an Observable which is merged in the output Observable, in a serialized fashion waiting for each one to complete before merging the next.

將source Observable里的每個元素施加一個projection函數，這個projection函數返回一個新的Observable，然后將所有這些Observable flatten成另一個Observable.

Returns an Observable that emits items based on applying a function that you supply to each item emitted by the source Observable, where that function returns an (so-called “inner”) Observable. Each new inner Observable is concatenated with the previous inner Observable.

例子：

const clicks = fromEvent(document, 'click');const ioe = interval(1000);const mapfn = take(4);const comp = ioe.pipe(mapfn);const fn = ev => comp;const result = clicks.pipe( concatMap(fn));result.subscribe(x => console.log('diablo: ' +x));

ioe是Observable：

mapfn是OperatorFunction：

comp是一個新的Observable：

fn函數接收任意類型的輸入變量，返回一個新的Observable：

測試結果：每點擊屏幕一次，順序打印0～4：

concatMap語義：每點擊一次，從interval Observable中拿出4個元素出來，生成4個新的Observable，然后連接在一起。

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