Basic Javascript (53/111) | Understanding Undefined Value returned from a Function | freeCodeCamp

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"A function can include the return statement but it does not have to. In the case that the function doesn't have a return statement, when you call it, the function processes the inner code but the returned value is undefined.

Example

var sum = 0;
function addSum(num) {
sum = sum + num;
}
addSum(3); // sum will be modified but returned value is undefined
addSum is a function without a return statement. The function will change the global sum variable but the returned value of the function is undefined.

Create a function addFive without any arguments. This function adds 5 to the sum variable, but its returned value is undefined."