CODEWITHSUNDEEP
javascriptarrayslistfunction
lsgng
lsgng

Reputation: 495

JavaScript: Checking a list for elements

I'm currently learning JavaScript using Marijn Haverbekes excellent book "Eloquent JavaScript". Now there is this exercise where you have to write a recursive function that returns the nth element of a nested list. If there is no such element, the function is supposed to return undefined. The solution looks like this:

function nth(list, n) {
  if (!list)
    return undefined;
  else if (n == 0)
    return list.value;
  else
    return nth(list.rest, n - 1);
}

So far, everything seems pretty clear to me. However, I don't really get what exactly if (!list) {} does. How exactly does this condition evaluate? Why is it true, if list has an element n?

The complete excercise can be found here: http://eloquentjavascript.net/04_data.html#p_7AnSuS26HF

Upvotes: 0

Views: 72

Answers (2)

Mulan
Mulan

Reputation: 135367

This

if (!list)

Is a shorthand way of saying

if (list === false || list === 0 || list === '' || list === null || list === undefined || list !== list /* NaN */) ...

!list will happen in when the list is shorter than n elements.

// 4 is larger than the list length here, (2)
// !list will happen
nth({value: 'a', rest: {value: 'b', rest: null}}, 4)
//=> undefined

// 1 is not larger than the list length
// !list will not happen
// instead, n === 0 happens after recursing 1 time
nth({value: 'a', rest: {value: 'b', rest: null}}, 1)
//=> 'b'

Upvotes: 2

gurvinder372
gurvinder372

Reputation: 68413

However, I don't really get what exactly if (!list) {} does.

It checks if list variable still has a value which is other than falsey value such as null, undefined, 0, false, NaN, "" or ''. If the list has any falsey value, then it returns undefined.

Why is it true, if list has an element n?

As per the link you have shared, value of list is

var list = {
  value: 1,
  rest: {
    value: 2,
    rest: {
      value: 3,
      rest: null
    }
  }
};

which means either rest has child elements or it has null.

eventually, it will have null which is what this condition will assert and return undefined.

Upvotes: 0

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