javascript how to use a variable to reference an object

Question

I have a javascript object like this

var obj={
    a:{x: "someValue", y:"anotherValue"},
    b:{x: "bValue", y:"anotherbValue"}
};

and I am trying to reference it like this

function(some_value){
    alert("some_value is " + some_value + " with type " + typeof some_value);
    // prints  some_value is a  with type  string 
    var t;
    t=obj[some_value]["x"];   // doesn't work   
    some_value="a";
    t=obj[some_value]["x"];  // this does work
    t=obj["a"]["x"];     // and so does this
}

I would really like understand what is going on here . Ideally I'd like to reference my object with the value passed to the function. Thanks

sorry there is a typo - there is not two double quotes on the last object in the real code — sferret
– sferret, Commented Apr 29, 2012 at 1:43
You can edit your question. stackoverflow.com/posts/10369028/edit — user1106925
– user1106925, Commented Apr 29, 2012 at 1:44
If some_value is indeed "a", then t=obj[some_value]["x"]; will work. — user1106925
– user1106925, Commented Apr 29, 2012 at 1:46

ChaosPandion · Accepted Answer · 2012-04-29 01:46:37Z

1

I can only assume that your variable some_value must not contain the value a. It is possible that it has extra white space characters.

answered Apr 29, 2012 at 1:46

ChaosPandion

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2 Comments

user1106925 Over a year ago

+1 Seems like it may be a whitespace issue judging by the output of the alert.

sferret Over a year ago

yes that was it- thanks, at least I won't forget to check for that again since I have been fiddling with it for ages.

Joseph · Accepted Answer · 2012-04-29 01:53:09Z

0

In JS, when a property does not exist, it returns an undefined. in the case of the following code, if the value contained in the variable some_value does not exist as a property in obj, t is undefined

//if some_value is neither a nor b
t = obj[some_value] // t === undefined

if you try to extract a property from an undefined value, the browser reports an error:

//if some_value is neither a nor b
t = obj[some_value]["x"] // error

you can check the existence of a property before you try accessing it by using hasOwnProperty().

if(obj.hasOwnProperty(somevalue)){
    //exists
} else {
    //does not exist
}

you can do a "loose check" but it's not reliable as anything "falsy" will call it "non-existent" even though there is a value.

if(obj[somevalue]){
    //is truthy
} else {
    //obj[somevalue] either:
    //does not exist
    //an empty string
    //a boolean false
    //null
    //anything "falsy"
}

edited Apr 29, 2012 at 1:53

answered Apr 29, 2012 at 1:47

Joseph

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1 Comment

sferret Over a year ago

thanks, I used your suggestion hasOwnProperty and found that it was returning false, some_value="my_lookup_value " Whitespace! Ahggg

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