Get index in an array.filter callback function

Question

I have the following array filter with a callback function:

array.filter(createPredicateFn(expression, comparator));

My call back function is declared as follow:

function createPredicateFn(expression, comparator) {

How can I get the index of the element inside my createPredicateFn?

Edit:

here is my predicate function:

     function createPredicateFn(expression, comparator) {
            var predicateFn;

            if (comparator === true) {
                comparator = angular.equals;
            } else if (!angular.isFunction(comparator)) {
                comparator = function (actual, expected) {
                    if (angular.isObject(actual) || angular.isObject(expected)) {
                        // Prevent an object to be considered equal to a string like `'[object'`
                        return false;
                    }

                    actual = angular.lowercase('' + actual);
                    expected = angular.lowercase('' + expected);
                    return actual.indexOf(expected) !== -1;
                };
            }

            predicateFn = function (item) {
                return deepCompare(item, expression, comparator);
            };

        }

We can't help you without the code for your createPredicateFn — Ivan Drinchev
– Ivan Drinchev, Commented Dec 5, 2014 at 15:05

Sirko · Accepted Answer · 2014-12-05 15:24:53Z

3

Within your createPredicateFn() you have to return another function. This inner function can have up to three parameters:

the value of the element
the index of the element
the Array object being traversed

(Source: MDN)

Hence the second parameter of your inner function is the index you look for.

A simplistic example could look like this:

function createPredicateFn(expression, comparator) {
  return function( val, index, arr ) {
    // do something and return a boolean here
    return index % 2 == 0;
  }
}

edited Dec 5, 2014 at 15:24

answered Dec 5, 2014 at 15:08

Sirko

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7 Comments

Nate Over a year ago

I tried that and get the index however, I can't make my function working (see my edit)

Sirko Over a year ago

Your given function never returns a function, but nothing (hence undefined). When you want to use a generator function, that generator has to return a function reference, so filter() knows, what function to use.

Nate Over a year ago

I'm a bit lost...Could you please rewrite my function with access to the index? I got this function from angularjs filter and I'm not very comfortable with it, that's why I'm not 100% sure of what I'm doing.

Sirko Over a year ago

@ncohen The actual function, that determines, whether an entry should stay in the array or get filtered is that predicateFn(), right? If so, just return this at the end of the function.

Nate Over a year ago

Got it, I will try that later today! Thanks

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