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js check if function exists

if (typeof yourFunctionName == 'function') { 
  yourFunctionName(); 
}

javascript check if function exists

if (typeof sourceObj.someMethod === "function") { 
    // you are safe using the method
  	sourceObj.someMethod();
}

js check if function exists

//check if myProcedure() function exists
if (typeof myProcedure === "function") { 
    // This function exists
}