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2. Working with Objects and Properties > Checking If an Object Contains a Given...
Checking If an Object Contains a Given Property Using the in Operator
The in operator is used to verify
(true or false) if an object contains a given property. Below, we are
checking to see if foo is a property in
myObject.
<!DOCTYPE html><html lang="en"><body><script>
var myObject = {foo: 'value'};
console.log('foo' in myObject); // logs true
</script></body></html>
You should be aware that the in
operator not only checks for properties contained in the object
referenced, but also for any properties that object inherits via the
prototype chain. Thus, the same
property lookup rules apply and the property, if not in the current
object, will be searched for on the prototype chain.
This means that myObject in the
above code actually contains a toString
property method via the prototype chain
(Object.prototype.toString), even if we
did not specify one (e.g., myObject.toString =
'foo').
