Javascript array iteration

Question

I have a JavaScript array in sessionStorage and when I do a sessionStorage.getitem('states'), I am getting like this:

"{
    "TamilNadu": [
        "Chennai", 
        "Trichy", 
        "Madurai"
    ],
    "Andhra": [
        "Hyderabad",
        "Vizhag"
    ],
    "Karnataka": [
        "Bangalore",
        "Mysore",
        "Darwad"
    ],
    "Maharashtra": [
        "Mumbai",
        "Pune"
    ]
}"

Now my requirement is to get the state name from the city name. For example if I pass "Mumbai" to a function as a parameter, that function should read this sessionstorage value and return me "Maharashtra".

Could you please help me how to achieve this?

Hello Manwal, i am pretty much a newbie to javascript programming. I have just started learning. So didnt try much as i was not sure what to do. — Shivashankar Balu
– Shivashankar Balu, Commented Jan 16, 2015 at 10:11
nit-pick: you don't have an array, you have a JSON string, that -when parsed- will yield an object. An object is not the same as an array — Elias Van Ootegem
– Elias Van Ootegem, Commented Jan 16, 2015 at 10:12
Thanks a lot for all your answers in a very short time. I am very much grateful. I tried the code by "Pavel" and "belhyk" and it is giving me the results as expected. Just one question. In future the json string grows a lot and contains so much of data, any of these answers given below should work, isnt it? — Shivashankar Balu
– Shivashankar Balu, Commented Jan 16, 2015 at 10:44
All answers will work. But if your json will grow, I suggest to look at performance (see my answer bellow). Good luck — Arūnas Smaliukas
– Arūnas Smaliukas, Commented Jan 16, 2015 at 11:17

Pavel Patrin · Accepted Answer · 2015-01-16 10:05:16Z

1

This code:

sessionStorage.getitem('states')

returns a string. You must parse it with:

var obj = JSON.parse(sessionStorage.getitem('states'))

And next access object attributes with square brackets or a dot:

for (var attr in obj) {
    if (obj.hasOwnProperty(attr) && obj[attr][0] == 'Mumbai') {
        return attr;
    }
}
return null;

answered Jan 16, 2015 at 10:05

Pavel Patrin

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1 Comment

Shivashankar Balu Over a year ago

Thanks a lot for all your answers in a very short time. I am very much grateful. I tried the code and it is giving me the results as expected. Just one question. In future the json string grows a lot and contains so much of data, any of these answers given below should work, isnt it?

belykh · Accepted Answer · 2015-01-16 10:17:15Z

1

function getStateByCityName (cityName) {
    var states = {};
    try {
        states = JSON.parse(sessionStorage.getitem('states'));
    } catch (e) {
    }
    for (var state in states) {
        for (var i = 0; i < states[state].length; i ++) {
            if (states[state][i] == cityName)
                return state;
        }
    }
    return null;
}

edited Jan 16, 2015 at 10:17

answered Jan 16, 2015 at 10:10

belykh

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6 Comments

DontVoteMeDown Over a year ago

How can you iterate over a string?

Pavel Patrin Over a year ago

You must do a JSON.parse

belykh Over a year ago

I have added parsing of a string

Strille Over a year ago

Adding a try...catch without really handling the error doesn't really do much good in my opinion. You just really "hide" the error, which means you cannot differentiate between when the states data could not be read for some reason and when you asked for a cityName not present.

belykh Over a year ago

try...catch for "JSON.parse" is a minimal handling for this function

|

Arūnas Smaliukas · Accepted Answer · 2015-01-16 11:21:08Z

Or simply use if (states[state].indexOf(cityName) !== -1)

getStateByCityName(cityName) {
    var statesRaw = sessionStorage.getitem('states');
    var states = JSON.parse(statesRaw);
    for (var state in states) {
        if (states[state].indexOf(cityName) !== -1) {
            return state;
        }
    }
    return false;
}

Or in EcmaScript 5 loop style :))

getStateByCityName(cityName) {
    var statesRaw = sessionStorage.getitem('states');
    var states = JSON.parse(statesRaw);

    Object.keys(states).forEach(function(state) {
        if (states[state].indexOf(cityName) !== -1) {
            return state;
        }
    })
    return false;
}

There are many ways (see other answers too) so let's take a look at performance.

First to iterate through your object http://jsperf.com/object-keys-vs-for-in-with-closure/3

In my browser fastest case was Object.keys for loop, so

getStateByCityName(cityName) {
    var statesRaw = sessionStorage.getitem('states');
    var states = JSON.parse(statesRaw);
    var states_keys = Object.keys(states);

    for (var i = 0, l = states_keys.length; i < l; i++) {
          // check if city exists
    }
    return false;
}

Next we need to check if city exists in that state. I see two ways to do it: 1) as others suggested iterate with for loop 2) mine suggested .indexOf()

http://jsperf.com/js-for-loop-vs-array-indexof/2 in my browser indexOf() got better results, so final function

getStateByCityName(cityName) {
    var statesRaw = sessionStorage.getitem('states');
    var states = JSON.parse(statesRaw);
    var states_keys = Object.keys(states);

    for (var i = 0, l = states_keys.length; i < l; i++) {
        if (states[states_keys[i]].indexOf(cityName) !== -1) {
            return states_keys[i];
        }
    }
    return false;
}

And as some one suggested use try/catch

getStateByCityName(cityName) {
    var states = {};
    try {
        states = JSON.parse(sessionStorage.getitem('states'));
    } catch (e) {
    }
    var states_keys = Object.keys(states);

    for (var i = 0, l = states_keys.length; i < l; i++) {
        if (states[states_keys[i]].indexOf(cityName) !== -1) {
            return states_keys[i];
        }
    }
    return false;
}

Thank you Arūnas Smaliukas for giving me this whole bunch of information on this. I will follow these.

Jairo · Accepted Answer · 2015-01-16 10:11:07Z

0

That should work but can't test it:

function (arrayTarget, itemToSearch){
    for (x in arrayTarget) {
        for(y in arrayTarget[x])
            if(itemToSearch == arrayTarget[x][y])
                 return x;   
    }
}

answered Jan 16, 2015 at 10:11

Jairo

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