About ' testable result ' in pure function

Question

In JavaScript there is a conception named pure function. One of its features is that it always returns a testable result. I am very confused with this. I'd like to know the definition and the goal and why of it.

Answer 1

A "testable result" is not a JS term. Is a physics term. When you said "Testable result" is when you make the same action and get the same result.

For example:

• If you throw a ball at the same distance it will always hit the ground at the same time
• If you drive 60km/h in 1 hour you will 60 km away.

So, in software, a testable result will be functions when you pass the same arguments you will get the same results.

For example:

• cos(x) // Allways the same
• max(a,b,c) // Allways the same
• uppercase(str) // Allways the same

Non-testable results:

• random(from, to) // Diff results
• uuid(str) // Diff results
• getNowTime(format) // Diff results

Now, the concept of pure-function is:

• Is a "testable result" (in other words: if you send the same args you will get the same result)
• Not use input or output streams
• Not use global variables (only local vars and arguments)
• Not change local static variables
• Not change any argument

In javascript examples of non-pure functions:

// Is not a testable result 
function notPure(max, min) {
    return Math.random() * (max - min) + min;
}

// Use output stream
function notPure(name) {
    const res = "hi " + name;
    console.log(res);
    return res;
}

// Use global variable [This is a testable result but not pure because global variable]
const globalVar = "jett";
function notPure(lastname) {
    return "hi " + globalVar + " " + lastname;
}

// In JS don't exist "local static variables" so is impossible to replicate this case

// Change an argument
function notPure(obj, key, val) {
    obj[key] = val;
    return obj;
}

Some examples of pure functions will be:

function pure(a, b) {
   return a + b;
}

function pure(name, lastname) {
   return "hi " + name + " " + lastname;
}

function pure(obj, key, val) {
   const res = JSON.parse(JSON.stringify(obj));
   res[key] = val;
   return res;
}

In some examples, you will see "If there is an HTTP request is not a pure-function" and this is because the HTTP request uses I/O streams and is a high possibility to get different results.

EDIT: The most important cases for pure functions is because they are "testable" so you can do unit tests without any concern and you will not get "side-effects" in your code.