Question 206552 in Javascript Question posted in
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Javascript – why array.reduce returns NaN for no reason

I simply do this:

const ar = [1, 2, 3];
ar.reduce(Math.max, 0); // returns NaN
ar.reduce((a, c) => Math.max(a, c), 0); // returns 3 as expected

Any reason for this behaviour?

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2

Answers


  1. 0

    The callback function to Array#reduce is passed 4 arguments: the accumulator, the current element, the index, and the original array. All of these end up being passed to Math.max.

    The array itself cannot be converted to a number, so Math.max returns NaN.

    ar.reduce(Math.max, 0); is essentially equivalent to the following code:

    const ar = [1, 2, 3];
console.log(ar.reduce((acc, curr, i, arr) => {
  console.log(acc, curr, i, arr); // only for demonstration
  return Math.max(acc, curr, i, arr);
}, 0));

    ar.reduce((a, c) => Math.max(a, c), 0) uses only the arguments that it needs, so it works correctly.

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  2. 0

    @Unmitigated has clearly explained what’s going on in your code and has shown how to use Array#reduce and especially Math#max.

    Here, I show you can use just Math#max:

    const 
      ar = [1, 2, 3],
      
      mx = Math.max(...ar);
      
console.log( mx );
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