How to check if an array contains a specific value?

i have an array like below:
[{"A":"a","B":"b"},{"A":"a","B":"b"},{"C":"c"}]

how to check if the array elements' key contains "A"?(only key)

<script type="text/javascript">
var theArray=[{"A":"a","B":"b"},{"A":"a","B":"b"},{"C":"c"}]
for (var i = 0; i < theArray.length; i++) {
var pos=0;
for(b in theArray[i]){
if(b=="A"){
alert('"A" found in object '+i+' at position '+pos)
}
pos++
}
}
</script>

<script type="text/javascript">
var theArray=[{"A":"a","B":"b"},{"A":"a","B":"b"},{"C":"c"}]
for (var i = 0; i < theArray.length; i++) {
var pos=0;
for(b in theArray[i]){
if(b=="A"){
alert('"A" found in object '+i+' at position '+pos)
}
pos++
}
}
</script>

what do u think of this one?
var arr = [{"A":"a","B":"b"},{"A":"a","B":"b"},{"C":"c"}] ;
for(var i=0;i<3;i++)
{
if(arr[i].A!=undefined)
alert('found A');
}

depending on the situation, the array iteration methods are also useful.
var has_A = arr.some(function (item) {
return ("A" in item);
});

if("A" in theArray)

if("A" in theArray)
looks cool, does it work?

It works, for me. I believe as long as it's an associative array, it will work for the key, not the value, I think.

you are aware that there is no such thing as an "Associative Array" in JavaScript? (that would rather be an Object, lacking all Array methods).

and though "A" in obj works for objects/associative arrays, it does not for the OP’s variable, which is an Array of Objects, i.e. a nested structure.