Explicit passing initial value as undefined in array reduce

Question

I was testing few scenarios with array reduce function.

[1,2].reduce(function(initial ,  val){

        return initial+val;
    } ,1)
    // returns 4 as expected


[1,2].reduce(function(initial ,  val){

    return initial+val;
})
// returns 3

But explicitly passing undefined as initial value

[1,2].reduce(function(initial ,  val){

    return initial+val;
}, undefined)
// returns NaN.

It appears unusual to me.

checkout the syntax of array reduce arr.reduce(callback[, initialValue]) where initial value is used as the first argument to the first call of the callback. — Basilin Joe
– Basilin Joe, Commented Aug 30, 2016 at 6:01
I have seen that . In first case it ignores the initial undefined however in second case it returns NaN. Let me add one more example for better clarity — ajaykumar
– ajaykumar, Commented Aug 30, 2016 at 6:03
it's because you are trying to add undefined with a number try [1,2].reduce(function(initial , val){ initial = initial?initial:0 return initial+val; }, undefined) — Basilin Joe
– Basilin Joe, Commented Aug 30, 2016 at 6:07
Your example is working but the confusion what is the actual difference between the second and third case as in both the cases initial has undefined. — ajaykumar
– ajaykumar, Commented Aug 30, 2016 at 6:13

robertklep · Accepted Answer · 2016-08-30 06:34:29Z

5

Explaination here:

Note: If initialValue isn't provided, reduce will execute the callback function starting at index 1, skipping the first index. If initialValue is provided, it will start at index 0.

So your second case is similar to this:

[2].reduce(function(initial, val){
    return initial+val;
}, 1)

answered Aug 30, 2016 at 6:34

robertklep

205k38 gold badges421 silver badges412 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Explicit passing initial value as undefined in array reduce

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related