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How can I check if a JavaScript function is defined?
To check if a JavaScript function is defined or not, checks it with “undefined”.
Example
You can try to run the following example to check for a function is defined or not in JavaScript −
<!DOCTYPE html>
<html>
<body>
<script>
function display() {
alert("Demo Text!");
}
if ( typeof(display) === 'undefined') {
document.write('undefined');
} else {
document.write("Function is defined");
}
</script>
</body>
</html>
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