Gorgi Kosev

code, music, math

Gorgi Kosev

code, music, math

@spion

Intuitive JavaScript array filtering function pt1

Sat Jul 07 2012

When dealing with large JSON arrays on the client side or in node.js, one of our tasks might be to filter them on the client side before displaying them. Array.filter exists since JavaScript 1.6, however it seems kinda dry: all filters must be functions, even some really common filters such as matching text with a regular expression, checking if a number is within a range, checking if an enumeration has a certain value. Consider the following:

var testArray = [
        {name:"John",  age: 40, children: 2,
            company: { name: "MegaCorp", employees: 200}},
        {name:"Sue",   age: 30, children: 1,
            company:{ name: "MegaCorp", employees: 200}},
        {name:"Mary",  age: 55, children: 3,
            company:{ name: "MegaCorp", employees: 200}},
        {name:"Jack",  age: 20, children: 0,
            company:{ name: "MiniCorp", employees: 100}}];

// Using a for loop
var filtered = [];
for (var k = 0; k < testArray.length; ++k) {
    var item = testArray[k];
    if (item.age == 40 && item.age == 30) filtered.push(item);
}

// Using Array.filter
testArray.filter(function(item) {
    return item.age == 40 || item.age == 30
}); // returns John

The Array.filter variant is around two times shorter than the for loop variant. It also looks much cleaner: the anonymous function is the filter which is called on each item to check if it should get through the filter or not. We can call Array.filter with various kinds of "filter functions". Should be good enough for all uses, right?

Not so, at least not when you have lots of filters and you want them to be as short as possible to write. Or if you want to combine multiple filter functions from various sources and the items in the data array are fairly complex.

Lets say that we have a complex filter function for the company object and a simple regex name filter from elsewhere and we need to combine them. We would have to write the following:

testArray.filter(function(item) {
    return /^J.+$/.test(item.name)
        &&  complexFilter(item.company); });

However, now we cannot easily replace complexFilter for the company with anotherComplexFilter. We have to write code - to write a different anonymous function and use it instead.

Now imagine having multiple different complexFilters. Soon enough you will write the following function

intiutiveFilterBeta = function(someArray, filters) {
    return someArray.filter(function(item) {
        for (var k = 0; k < filters.length; ++k) {
            if (!filters[k](item)) return false;
        }
        return true;
    }
}

which will enable you to combine different complex filters into a filter array, essentially implementing the and operator.

At about this point you will probably realize that you are missing the or operator. What if you wish to filter all companies which complexCompanyFilter1 or complexCompanyFilter2 ? If you are like me, right now you are probably working on a DSL (domain specific language) in your head, a DSL which reminds you of SQL. You might also start thinking that this is going a bit over the top.

However, if you look closely you will notice certain peculiarity about the and operator: you do not really need to use and on two or more filters which are working on the same field. For example, you might want to match 1 or 2 children, but never both 1 and 2 children - it just doesnt make sense. You might also want to have a "between" filter for age, but you would not exactly want to and two between filters. Instead of between 30 and 50 and between 40 and 60 you would simply write a between 40 and 50 filter.

This observation seems to hold true for all primitive values except for strings. That doesnt really matter because we can easily filter strings with a tool made to do exactly that: regular expressions.

I decided to try and make a hopefully intuitive and readable yet still powerful filter function based on the observations above. It should enable some common primitive tests to be easily written without writing new functions. It should support the AND and OR operators intuitively and without writing functions in the most common cases. Finally, it should still enable writing custom filter functions. I came up with this:

function intuitiveFilter(array, filter) {
    var itemFilter = function (iFilter, item) {
        if (iFilter instanceof Function) {
            return iFilter(item);
        }
        else if (iFilter instanceof Array) {
            for (var k = 0; k < iFilter.length; ++k) {
                if (itemFilter(iFilter[k], item)) return true;
            }
            return false;
        }
        else if (typeof(item) == 'string' && iFilter
            && iFilter.test && iFilter.exec) {
            return iFilter.test(item);
        }
        else if (item === item + 0 && iFilter
            && (iFilter.lt || iFilter.gt || iFilter.le
            || iFilter.ge)) {
            // item is number and filter contains min-max
            return ((!("lt" in iFilter) || item <  iFilter.lt)
                &&  (!("gt" in iFilter) || item >  iFilter.gt)
                &&  (!("le" in iFilter) || item <= iFilter.le)
                &&  (!("ge" in iFilter) || item >= iFilter.ge));
        }
        else if (typeof (iFilter) === "object") {
            for (var key in iFilter) {
                if (!itemFilter(iFilter[key], item[key]))
                    return false;
            }
            return true;
        }
        return (iFilter == item);
    };
    var filtered = [];
    for (var k = 0; k < array.length; ++k) {
        if (itemFilter(filter, array[k]))
            filtered.push(array[k]);
    }
    return filtered;
}

And here are some neat ways to use it:

var testArray = [
        {name:"John",  age: 40, children: 2,
            company:{ name: "MegaCorp", employees: 200}},
        {name:"Sue",   age: 30, children: 1,
            company:{ name: "MegaCorp", employees: 200}},
        {name:"Mary",  age: 55, children: 3,
            company:{ name: "MegaCorp", employees: 200}},
        {name:"Jack",  age: 20, children: 0,
            company:{ name: "MiniCorp", employees: 100}}
];

console.log(intuitiveFilter(testArray,
    {name:/J.+/, age: {lt: 30}})); // Jack, but not John
console.log(intuitiveFilter(testArray,
    {age: [{gt: 15, le: 20}, {gt: 50}]})); // Jack and Mary
console.log(intuitiveFilter(testArray,
    {children: [0,1]})); // Jack, Sue

console.log(intuitiveFilter(testArray,
    {company: {name: "MegaCorp"}})) // all except Jack
console.log(intuitiveFilter(testArray,
    {age: function(a) { return a % 10 == 0 }})); // all except Mary
console.log(intuitiveFilter(testArray,
    [{age: 30 }, {company:{name:"MiniCorp"}}])); // Sue and Jack

The function is designed to make most filters look like a part of an item from the array that is being filtered. The examples demonstrate some possible uses.

In the first example-set, the first one is a classic and operator with a regex and a numeric operator for age. The second example showcases the simple numeric support. The third example is the purest form of the or operator on the number of children. Similar filters could easily be written for the string name with regular expressions, for example: {name:[/M.+/, /S.+/]}. Isnt that concise and lovely?

In the second set, the example {company: {name: "MegaCorp"}} showcases the ability of the filter to go deeper in the object. The second example shows the ability of the filter to use custom functions anywhere. The last example demonstrates the ability to use the or operator on filters which work on different fields.

The function would've been perfect if it wasn't for a caveat: it cannot check into arrays the same way it can check into an object. For example, if we had the following data:

var arrayArray = [{name:"John",  age: 40,
    children: [{name:"Joe", age:12}, {name:"Jane", age:10}],
    company:{ name: "MiniCorp", employees: 100}}]

we wouldn't have a way to test the contents of the sub-array children without writing a function:

intuitiveFilter(arrayArray, {children: function(arr) {
    return childrenArrayFilter(arr, {age:{gt:10}}).length > 0; }
});

This caveat isnt hard to fix. However, I have decided that I will leave it unfixed for now: let the fix be an exercise for the reader. If this code generates some interest I will supply my fix later. The fix can even be added without modifying the original function.