Have you ever run a function that works and the tester returned it to you with a comment that it is untestable or doesn't apply to clean code concepts?

while going throw some coding challenges, found concept of pure function and impure function.

Pure function

In simple terms, pure functions do not have an internal state. Therefore, all operations performed in pure functions are not affected by their state. As a result, the same input parameters will give the same deterministic output regardless of how many times you run the function.

To get a better understanding, let’s consider the following example.

function add(a,b) { 
  return a + b
}
console.log(add(4,5))

This example contains a simple add() function, which gives 9 as the output. It is a very predictable output, and it does not depend on any external code. This makes the add() function a pure function.

If a function is declared pure and does not have a state, it can share many instances inside a class. Also, it is advised to avoid mutations inside pure functions.

Advantages of pure functions

• A pure function works as an independent function that gives the same output for the same inputs.

• Pure functions are readable because of independent behavior. Moreover, they are straightforward to debug.

• You can clone an external state into a pure function, but it does not change the purity of the function.

Impure Functions

An impure function is a function that contains one or more side effects. It mutates data outside of its lexical scope and does not predictably produce the same output for the same input.

For example, consider the following code snippet:

var addNew = 0;
function add(a,b){ 
  addNew =1; 
  return a + b + addNew
}
console.log(add(4,5))

In the above example, there is a variable named addNew, and it is declared outside of the add() function. But the state of that variable is changed inside the add() function. So, the add() function has a side effect on a variable outside of its scope and is therefore considered an impure function.

In general, it’s ideal to keep the impure elements of your programs distinct from the data processing, which is usually pure. Also, updating and maintaining your applications will be much easier if you confine impure elements to their particular functions.

The following JavaScript functions are inherently impure:

• Math.random() is an impure function since it modifies the internal state of the Math object and provides different results with each call. So Math.random() can contain side effects.

Math.random(); (Output: 0.4450692005082965)

In the above code snippet, no arguments pass into any of the Math.random() function calls, but still they all produce a different result.

• Date.now()
• arr.splice()
• arr.push()
• arr.sort()
• console.log() and alert() are also impure functions (although they generate the same behavior and always return the same value for identical calls).
• JavaScript is synchronous by its nature. Therefore, asynchronous functions such as fetch and promise are impure.

Advantages of impure functions

• Impure functions can use an in-place solution to reduce the space complexity.
• In impure functions, the state can be modified to use the parent variable and call for the function compiling.

As mentioned before, the main difference between pure and impure functions in JavaScript is side effects. So, let’s discuss some specifics about the side effects.

Side effects

Side effects can occur in your program when it uses an external code block inside a function. As a result, there can be performance issues in your application. for example:

var preNumber =2;
function addValue(newNumber){ 
  return preNumber += newNumber; 
}

In the above example, the variable preNumber is used inside the addValue() function. This behavior can result in the following side effects.

Side effect 1: Dependency issue

The addValue() method depends on the preNumber variable. If preNumber is not defined or not available, the method will throw an error.

Side effects 2: External code modification

When the addValue() function executes, it changes the state of the preNumber variable. It shows that the addValue() method has a side effect of modifying external code.

Side effects 3: Non-deterministic function

The addValue() function uses external code. It makes the function non-deterministic, which means you cannot determine the output by looking.

Conclusion

• Pure functions do not have side effects. Impure functions can cause side effects.
• Pure functions return the same output if we use the same input parameters. However, impure functions give different outcomes when we pass the same arguments multiple times.
• Pure functions always return some results. Impure functions can execute without producing anything.
• Debugging pure functions is relatively easier than debugging impure functions.
• Pure functions cannot execute AJAX calls or standard DOM manipulation.
• Impure functions aren’t inherently wrong. They merely can cause some confusion in more extensive systems in the form of spaghetti code.