Have you ever used the comma operator?

What is the comma operator ？
mdn explain ： Evaluate each of its operands （ From left to right ）, And return the value of the last operand .
translate ： Such as （x,y）, Calculate from left to right x and y, Finally back to y
mdn Case study ：

let x = 1;
x = (x++, x);
console.log(x); // 2 

Does it feel like this comma operator doesn't work ？ Existence makes sense , It is still useful in some scenes .

Take a chestnut ：（ use reduce Can achieve reverse）

 var arr = [1,2,3,4,5]
 var result = arr.reduce((t,v)=> (t.unshift(v),t),[])
 console.log(result) // [5,4,3,2,1]

reduce What is it again? ？
mdn explain ：reduce() Method to execute a reducer function ( Ascending execution ), Summarize its results into a single return value .
I'd better translate it ： ok , Unable to translate , Anyway, it's the function of accumulator
reduce(func,initValue)
Parameters ：
func: Callback function , And there are four parameters
acc: accumulator ( To put it bluntly, you can get the last return value )
cur: Current value
idx: Current index
oarr: Original array

initValue: Initial value

Let's take a look at the chestnut just now ：

 var arr = [1,2,3,4,5]
 var result = arr.reduce((t,v)=> (t.unshift(v),t),[])
 console.log(result) // [5,4,3,2,1]

I may ask if it's good to do this directly ：var result = arr.reduce((t,v)=> t.unshift(v),[])
Come on, read the newspaper wrong ：Uncaught TypeError: t.unshift is not a function
unshift Method returns the length of the array , So next time t Namely number Type instead of array , So this error will be reported .

Take apart this writing method is ：

var result = arr.reduce((t,v)=> {
    
t.unshift(v);
return t;
},[])

Then the operation of the comma operator can simplify the writing ：

 var result = arr.reduce((t,v)=> (t.unshift(v),t),[])