结合数组长度，为什么返回NaN？ 的JavaScript

Question

Info: Hi you can see there are 4 arrays, and then an array with those arrays inside. I'm using _.reduce to add up the length of the arrays. I'm getting NaN when I call the function with the array of arrays. I would expect to get 19.

Why is this? Thanks for help

var array1 = [[1],[2],[3],[4],[5]];
var array2 = ["a", "b"];
var array3 = [{name : "John"}, {age : 21}];
var array4 = [1,2,3,4,5,6,7,8,9,0];

var array5 = [array4, array3, array2, array1];

var arrayComboLength = function(array){
return _.reduce(array, function(a,b){
    return a.length + b.length;
});
}


console.log(arrayComboLength(array5));  --> // NaN

Answer 1

The first argument of reduce 's callback is the accumulator. You are accessing the length of a number and that results in undefined . And then you are adding undefined to the numbers that are the lengths of the rest of the arrays which results in undefined .

var arrayComboLength = function(array) {
  return _.reduce(array, function(a, b) {
    return a + b.length;
  }, 0);
}

This is very close to the example given in the documentation http://underscorejs.org/#reduce

Answer 2

The counting function will be recursively applied to all elements of the array. Recursively, so they will also be applied to the scalar values in array4, for example. Scalar values don't have a length.