Question

I’m trying an obvious task:

var maxVal = [ 1, 2, 3, 4, 5 ].reduce( Math.max, 0 );

and get:

NaN

as the result. To make it work I have to make an anonymous function this way:

var maxVal = [ 1, 2, 3, 4, 5 ].reduce( function ( a, b ) { 
                                           return Math.max(a, b);
                                       }, 0 );

Could someone tell me why? Both are functions that take two arguments and both return one value. What’s the difference?

Another example could be this:

var newList = [[1, 2, 3], [4, 5, 6]].reduce( Array.concat, [] );

The result is:

[1, 2, 3, 0, #1=[1, 2, 3], #2=[4, 5, 6], 4, 5, 6, 1, #1#, #2#]

I can run this example in node.js only under this shape (Array has no concat in node.js v4.12, which I use now):

var newList = [[1, 2, 3], [4, 5, 6]].reduce( [].concat, [] );    

and then get this:

[ {}, {}, 1, 2, 3, 0, [ 1, 2, 3 ], [ 4, 5, 6 ], 4, 5, 6, 1, [ 1, 2, 3 ], [ 4, 5, 6 ] ]

And why is that?

Answer 1

The function passed to reduce takes more than 2 arguments:

• previousValue
• currentValue
• index
• array

Math.max will evaluate all arguments and return the highest:

Math.max(1, 2, 3, 4) === 4;

So in case of passing Math.max to reduce, it will return the highest value from the 4 arguments passed, of which one is an array. Passing an array will make Math.max return NaN because an array is not a number. This is in the specs (15.8.2.11):

Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.

…

If any value is NaN, the result is NaN

ToNumber will return NaN for an array.

So reducing with Math.max will return NaN in the end.