_.indexOf() `item !== item` - what is its purpose?

Question

Looking into Underscore.js code and more specifically into _.indexOf() function (find code with comments here)

_.indexOf = function(array, item, isSorted) {
    var i = 0, length = array && array.length;
    if (typeof isSorted == 'number') {
      i = isSorted < 0 ? Math.max(0, length + isSorted) : isSorted;
    } else if (isSorted && length) {
      i = _.sortedIndex(array, item);
      return array[i] === item ? i : -1;
    }
    if (item !== item) {
      return _.findIndex(slice.call(array, i), _.isNaN);
    }
    for (; i < length; i++) if (array[i] === item) return i;
    return -1;
};

I noticed the if(item !== item){...} statement, but I don't get its purpose. items is a parameter and it's not changed inside the function. When would a variable differ from itself?

Am I missing something?

this is for the the future me: http://ecma-international.org/ecma-262/5.1/#sec-15.1.2.4 — BeNdErR, Mar 10 '15 at 16:17

score 4 · Accepted Answer · edited May 23 '17 at 11:56

4

IEEE-754 NaNs are not equal to themselves. The if statement is checking whether item is a NaN. If it is, the function needs to use special logic since the search loop testing for array[i] === item won't work.

For further discussion, see Why is NaN not equal to NaN? and What is the rationale for all comparisons returning false for IEEE754 NaN values?

edited May 23 '17 at 11:56

Community

1
1

answered Mar 10 '15 at 14:39

NPE

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score 2 · Answer 2 · answered Mar 10 '15 at 14:39

The number constant NaN is never === to another value, including itself. Thus that's a way to test for NaN without a function call. Absolutely any other value of item would test equal to itself:

undefined is === to undefined
null is === to null
a number is equal to itself, as is a string or a boolean
a reference to an object is === to itself (and only to itself!)

_.indexOf() `item !== item` - what is its purpose?

2 Answers2