Side Effects & pure functions

Question 1

What actions qualify as a function side effect?

Answer 1

1. It reassigns any non-local variable.
2. It mutates the value of any object referenced by a non-local variable.
3. It reads from or writes to any data entity (files, network connections, etc.) that is non-local to your program.
4. It raises an exception.
5. It calls another function that has any side effects that arenotconfined to the current function. For instance, if you call a function that mutates an argument, but that argument is local to the calling function, then it isn’t a side effect. (See the example withsortlater in this assignment.)

Question 2

What side effect does this have?

let number = 42;
function incrementNumber() {
number += 1;
}

Answer 2

side effect: reassigning ‘number’ defined in outer scope

Question 3

What side effect does this have?

let letters = [‘a’, ‘b’, ‘c’];
function removeLast() {
letters.pop();
}

Answer 3

// side effect: alters the array referenced by letters

Question 4

What IO actions count as side effects?

Answer 4

• Reading from a file on the system’s disk
• Writing to a file on the system’s disk
• Reading input from the keyboard
• Writing to the console
• Accessing a database
• Updating the display on a web page
• Reading data from a form on a web page
• Sending data to a remote web site
• Receiving data from a remote web site
• Generating random numbers with Math.random()
• Interacting with devices that change the system state in such a way that is observable, such as:
  
  • The mouse, trackpad, or other pointing devices
  • The random number generator
  • The audio speakers
  • The camera

Question 5

What side effect does this have?

function divideBy(numerator, denominator) {
if (denominator === 0) {
throw new Error(“Divide by zero!”);
}

return numerator / denominator;
}

Answer 5

// side effect: raises an exception

Question 6

how does exception catching impact side effects?

Answer 6

function divideBy(numerator, denominator) {
try {
if (denominator === 0) {
throw new Error(“Divide by zero!”); // side effect: raises an exception
}
} catch (error) {
console.log(error.message);
return undefined;
}

return numerator / denominator;
}

Question 7

What are pure functions?

Answer 7

Pure functionsare functions that:

1. Have no side effects.
2. Given the same set of arguments, the function always returns the same value during the function’s lifetime. This rule implies that the return value of a pure function depends solely on its arguments.