JavaScript undefinedProperty
JavaScript Global Functions
Definition and Usage
The undefined property is used to store the undefined value in JavaScript.
Browser Support
All major browsers support the undefined property.
Example
Example
Detect whether a variable is defined:
<script>
var t1="";
var t2;
if (t1===undefined)
{
alert("t1 is undefined");
}
if (t2===undefined)
{
alert("t2 is undefined");
}
</script>
var t1="";
var t2;
if (t1===undefined)
{
alert("t1 is undefined");
}
if (t2===undefined)
{
alert("t2 is undefined");
}
</script>
The above example outputs:
t2 is undefined
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