converting string into comparison operator. Possible or not? Javascript

Question

I'm creating a function to filter my javascript objects.

function filter(array,filterName,filterCondition,filterParameter){

    for(var i=0; i< array.length;i++){
        if(array[i][filterName] -<convert FilterCondition here>- filterParameter){

        }
    }

}

and ideally I would like to use it like this:

filter(anArray,"age",">",10)

Is it possible to convert a string comparison operator into a real operator in my if statement?

@blex thanks this works! If you want to provide answer I will accept — Terence Chow
– Terence Chow, Commented Jun 26, 2015 at 21:16
Thanks Chowza. If you want to avoid using eval or a big switch statement, I've found an alternative way, check my edited answer. — blex
– blex, Commented Jun 26, 2015 at 22:00

cassln · Accepted Answer · 2015-06-26 21:30:40Z

8

For example, you can use hash like this:

function filter(array,filterName,filterCondition,filterParameter){
    var comparisonOperatorsHash = {
        '<': function(a, b) { return a < b; },
        '>': function(a, b) { return a > b; },
        '>=': function(a, b) { return a >= b; },
        '<=': function(a, b) { return a <= b; },
        '==': function(a, b) { return a == b; },
        '===': function(a, b) { return a === b; },
    };

    var comparisonOperator = comparisonOperatorsHash[filterCondition];

    if (comparisonOperator === undefined) {
        // throw error here
    }

    for(var i=0; i< array.length;i++){
        if(comparisonOperator(array[i][filterName], filterParameter)) {

        }
    }
}

answered Jun 26, 2015 at 21:30

cassln

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3 Comments

Walter Chapilliquen - wZVanG Over a year ago

It is the same logic of other answer, why repeat it again?

cassln Over a year ago

Hash is more handy, than switch. And allow to parametrize function without editing function body.

Terence Chow Over a year ago

I've decided to change the accepted answer to this one as I find this to be easier to use, ie you can also use it to compare strings etc

blex · Accepted Answer · 2015-06-26 22:16:13Z

eval is evil, but can do anything. It's considered evil if arbitrary input can be passed to it, like user input. But if you control what's in there, it should be safe. More info here.

So you could do:

function filter(array,filterName,filterCondition,filterParameter){
    var results = [];
    for(var i=0; i< array.length;i++){
        if(eval(array[i][filterName] + filterCondition + filterParameter)){
            results.push(array[i]['name'])
        }
    }
    document.body.innerHTML = 'These guys are older than 10: ' + results.join(', ');
}

var anArray = [
  {'name': 'Patrick', 'age': 8 },
  {'name': 'John', 'age': 12 },
  {'name': 'Debora', 'age': 26 },
  {'name': 'Jenna', 'age': 3 },
  {'name': 'Brandon', 'age': 14 },
];

filter(anArray,"age",">",10);

Edit

I've thought of another way without the eval, or an endless switch statement. The trick here is to create a temporary script tag, that will hold a function to check conditions. This will avoid recompiling your function on every iteration of the loop, as it would with eval.

var conditionScript = document.createElement('script');

function filter(array,filterName,filterCondition,filterParameter){
    var results = [];
    prepareCondition(filterCondition,filterParameter);

    for(var i=0; i< array.length;i++){
        if( evalCondition(array[i][filterName]) ){
            results.push(array[i]['name']);
        }
    }

    document.body.innerHTML = 'These guys are older than 10: ' + results.join(', ');
}

function prepareCondition(filterCondition,filterParameter){
    if(conditionScript.parentNode) document.body.removeChild(conditionScript);
    conditionScript.innerText = 'function evalCondition(value){'
                              + '    return value ' + filterCondition + filterParameter
                              + '}';
    document.body.appendChild(conditionScript);
}

var anArray=[
  {'name': 'Patrick', 'age': 8 },
  {'name': 'John', 'age': 12 },
  {'name': 'Debora', 'age': 26 },
  {'name': 'Jenna', 'age': 3 },
  {'name': 'Brandon', 'age': 14 },
];

filter(anArray,"age",">",10);

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