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, Jun 26 '15 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, Jun 26 '15 at 22:00

score 8 · Accepted Answer · answered Jun 26 '15 at 21:30

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 '15 at 21:30

cassln

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It is the same logic of other answer, why repeat it again? – Walter Chapilliquen - wZVanG Jun 26 '15 at 21:31
Hash is more handy, than switch. And allow to parametrize function without editing function body. – cassln Jun 26 '15 at 21:35
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 – Terence Chow Jun 29 '15 at 14:55

blex · Answer 2 · 2015-06-26T22:16:13.507

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);

converting string into comparison operator. Possible or not? Javascript

2 Answers2