在字符串上实现 array.prototype.reduce()

Question

I have some confusion regarding how reduce operation takes place on a string.First a new Str instance is created and sends desired string as a parameter.

Then using split method it splits into an array of string.A reduceIt method takes the array and execute a reduce operation which returns the array element which has height length.

It works fine with a two element array.But if there is more than two elements it returns NAN.

Why it returns NAN for array having more than two elements??

function Str(text){
   this.text=text.split('\n');
}
Str.prototype.reduceIt = function() {
  return this.text.reduce(function(first,last,index,arr) {
          return Math.max(first.length,last.length);

  });
};

var t=new Str('i am a boy\n she loves cats\n a goat ate my flower garden ');
console.log(t.reduceIt());

Answer 1

The first time the callback is called first is a string (the first element in the array), and your function makes sense when first and last are both strings, so it works when the callback is only called once (the array has at most 2 elements).

The second time it is called it is the result of the previous call, a number . When you call first.length on a number you get undefined and when you call Math.max on that you get NaN .

If you want to find the length of the longest string in your array, you could use:

Math.max.apply(Math, this.text.map(function (str) { return str.length; }));

Answer 2

Some good answers already. :-)

The simple way to fix your problem is to supply an initial value of 0, then compare the returned value with the length of the new string, so:

Str.prototype.reduceIt = function() {
  return this.text.reduce(function(first,last,index,arr) {

          // Compare current value (first) with length of string
          return Math.max(first,last.length);

  }, 0); // supply 0 as the initial value
};

It might make things clearer to rename first to maxSoFar and last to currentString .

Answer 3

Why it returns NAN for array having more than two elements??

Because number.length is undefined , let's name your function foo and follow how it's invoked

1. foo(0, "i am a boy") gives NaN
2. foo(NaN, " she loves cats") gives NaN
3. foo(NaN, " a goat ate my flower garden ") gives NaN

Giving a final result of NaN .

This happens because number.length is undefined and Math.max(undefined, x) is NaN

It looks like you wanted to write a function which only takes the length of the second arg

function foo(a, b) {
    return Math.max(a, b.length);
}

In this case, you'll get

1. foo(0, "i am a boy") gives 10
2. foo(10, " she loves cats") gives 15
3. foo(15, " a goat ate my flower garden ") gives 29

Giving a final result of 29 .

Answer 4

Surprisingly reduce works natively with strings or any other array-like objects.

Array.prototype.reduce.call('String payload', ...);

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