javascript check if the attribute of an object exists

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javascript check if the attribute of an object exists

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I have an object that I get, and sometimes it comes with an attribute other times not, my problem is in the sequence when I go to work with it see the example below:

if( json.aviso != ""  ){ 
                    //...fazer alguma coisa 
                }

When the object does not come with the json.aviso attribute it performs what is inside if , I believe because null is different from "" would have a more elegant way of handling this do as:

if( json.aviso != "" && json.aviso !=null ){  //...me parece gambiarra }

javascript

asked by anonymous 20.10.2015 / 15:38

3 answers

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score 3 · Answer 1

There are 3 situations you need to pay attention to:

The attribute does not exist or its value is undefined ;

The attribute exists and its value is null ;

The attribute exists and its value is "falsy" ( false , 0 , "" , etc).

The most guaranteed way to differentiate between cases is to use the === and !== operators, since they take the type of the operands into consideration in addition to their values, while == and != have comparison rules which are not always intuitive.

Some examples:

var obj = { a:"foo", b:"", c:42, d:0, e:true, f:false, g:null, h:undefined };

[].forEach.call("abcdefghi", function(prop) {
    document.body.innerHTML += "<p>Testando: " + prop + " (" + obj[prop] + ")</p>";

    testa(prop, " === undefined");
    testa(prop, " === null");
    testa(prop, " === false");
    testa(prop, ' === ""');
    testa(prop, ' === 0');

    testa(prop, " == undefined");
    testa(prop, " == null");
    testa(prop, " == false");
    testa(prop, ' == ""');
    testa(prop, ' == 0');
});

function testa(prop, expr) {
    if ( eval("obj['" + prop + "'] " + expr) )
        document.body.innerHTML += "<p>obj." + prop + " " + expr + "</p>";
}

My suggestion is to list the cases where your condition applies and combine them with && , just like you are doing. If it is gambiarra or not, I can not say, but if it is, it is a matter of necessity imposed by the language ...

P.S. You may be wondering: "but how to differentiate between the case where an attribute does not actually exist and the case in which it exists but its value is undefined ?". Well, I'm asking myself the same thing, but I do not remember there being a way to do that.

In case of variables until it gives (see the question "How to verify undefined correctly in Javascript" for more details), but in case of object properties, I'm afraid not to (even converting the object to JSON will cause existing properties but with undefined to disappear from the result - since JSON does not support undefined ). / p>

score 2 · Answer 2

You can do it this way:

if ((json.hasOwnProperty("aviso")) && (!!json.aviso))

In this way you will first check if the object whose name is json exists the property aviso and lastly it will check the value of it, this way you guarantee that the existence of the property and its value is being verified. >

Edit:

There is a more effective way though that is to just leave !!json.aviso

if (!!json.aviso)

score 0 · Answer 3

if(json.aviso) {
   //code
}

However, if the value of your warning is equal to "" , it will not fall on your if

It is also worth noting that: This solution is only 100% valid in cases where the value contained within the warning is not numeric, because when the value is numeric and it has a value of 0 the code inside the if clause is not executed