Rxjs Observable.pipe passes in the execution logic analysis of multiple operators

Test code:

fromEvent(this.test, 'click').pipe(map( event => event.timeStamp), mapTo(1)).subscribe((event) => console.log(event));

Two input operations of the pipe operation:

The input parameter is an array containing two elements:

Using the original Observable as input, perform these two operations:

The reduce of an array is a native method:

The following code shows the usage of reduce:

<html>
<script>

var a = [1,2,3,4];

function fn(pre, cur, index, arr){
    console.log(`previous: ${pre}, cur: ${cur},
        index: ${index}, whole arr: ${arr}`);
    return pre + cur;
}

console.log(a.reduce( fn, 0));

</script>
</html>

reduce accepts two parameters, the first parameter is a function, the function receives 4 input parameters, previous, current, index and array:

previous: The previous round of reduce iteration value, if the first round, this value is the second parameter, which is the initial value
current: The current round of reduce iteration value. For example, if the elements of array are 1, 2, 3, 4, then each iteration of reduce, the current value is 1, 2, 3, 4, respectively.
index: the index value of the iteration
array: The original array of the call to reduce, with the previous index parameter, you can access the contents of the entire array