Simple example of .reduce()

Seek the output of [1,2,3].reduce((a,b) => console.log(a,b))?

The reason for writing this article is to see ↑ this topic, the print result is

4 5undefined 1undefined

At that time, I was very puzzled how to print the strange result of undefined
Let's take a look at the definition of reduce:

Definition and usage
The reduce() method accepts a function as an accumulator, and each value in the array (from left to right) is reduced to a value.
reduce() can be used as a higher-order function to compose functions.
Note: reduce() will not execute the callback function for an empty array.

Syntax
array.reduce(function(total, currentValue, currentIndex, arr), initialValue)

--------------------------------------------The above is the introduction of reduce------------------------------

You can see some examples below

Code block one:

[4,5,1].reduce((a,b) => console.log(a,b);)console result ==>4 5undefined 1undefined

Code block two:

[4,5,1].reduce((a,b) => {console.log(a,b);return a+b;})console result ==>4 59 110

Code block three:

========> 1:[4,5,1].reduce((a,b) => {console.log(a,b);return a+b;},0)console result ==>0 44 59 1=======> 2:[4,5,1].reduce((a,b) => {console.log(a,b);return a+b;},5)console result ==>5 49 514 1

It can be seen that in the code block 1 that prints undefined, there is no return, and in the reduce syntax, if you want to add, you need to set the return value –>total |Required.The initial value, or the return value after the calculation is complete .Since there is no return operation in this code block, the value of undefined will appear

The difference between code block 2 and code block 3-1 is that the initialValue parameter is added to the third, and the function of this parameter is to set the value passed to the function by Initial value , when not set, initialValue will start from the subscript value with index 1 (such as code block 2)

The difference can also be seen in code block three-1 and code block three-2, when initialValue is set to 0, it has no effect on the result of the data, and when it is set to 5, that is, set 5 as the initial value for the addition operation

Of course, in addition to simple mathematical operations such as addition, reduce has more advanced usage methods, such as array deduplication, array merging, etc. The following are the portals of other bloggers:
Other advanced operations of reduce