What is a utility function?

Definition

You may have heard the term utility function before but you may not understand exactly what a utility function is. A utility function is a function that has certain properties:

1) A utility function must not have side effects. A side effect in a function is any programming operation that accesses or modifies any state. A side effect might include:

• logging to the console
• changing the value of some variable
• writing some data to the disk
• dynamically adding a new element to the DOM

Basically: side effect = changing something somewhere.

let num = 2;

// wrong
function notUtilityFunction(x)
{
	console.log("hello, this is a side effect"); // side effect
	num = 4; // side effect
	localStorage.setItem("num", num); // side effect
}

// correct
function utilityFunction(x)
{
	return Math.pow(x,3); // not manipulating any state
}

2) A utility function’s output must depend directly on it’s input. A utility function usually takes in some value A and returns some value B (without changing A in the process)! B must directly depend on what A is and B must not rely on any random factors or operations that could produce a different result the next time the function is ran. For every A that is passed in to a utility function, there is exactly one B.

// wrong
function notUtilityFunction(A)
{
    return A * Math.random() * 3; // Result is not directly dependent on 'A'.
    // Return value will change depending on the random number that is generated
}
notUtilityFunction(3); // ? result depends on Math.random()


// correct
function utilityFunction(A)
{
    return A * 3; // Result is directly dependent on 'A'.
    // No matter what A you pass in, the same value will be returned (A*3)
}

utilityFunction(9); // ALWAYS 27

Summary

A utility function is a standalone function that does not have any side effects and whose output is directly dependent on it’s input. Utility functions perform common and often reused routines.