Create a function that only executes once

Problem

You want to be sure that the logic inside a function is executed only once.

Ingredients

• a self-defining function

Directions

1. Given: a function that is called twice, but should not do anything the second time it is called.

   function printSomething() {
     console.log('something');
   }
   printSomething();            // (1) Ok
   printSomething();            // (2) Should not be allowed

2. Add a variable to memorize if the function has been called.

   function printSomething() {
     let called = false;
     console.log('something');
   }

3. If it is false, then return undefined.

   function printSomething() {
     let called = false;
     if(called) {
       return undefined;
     }
     console.log('something');
   }

4. … otherwise set the variable to true …

   function printSomething() {
     let called = false;
     if(called) {
       return undefined;
     } else {
       called = true;
     }
   }

5. … execute the remaining logic of the function …

   function printSomething() {
     let called = false;
     if(called) {
       return undefined;
     } else {
       called = true;
       console.log('something');
     }
   }

6. … and overwrite the function to do nothing.

   function printSomething() {
     let called = false;
     if(called) {
       return undefined;
     } else {
       called = true;
       console.log('something');
       printSomething = function() {};
     }
   }

7. Voilá, now if printSomething() is called a second time, it simply does nothing.

   printSomething();        // (1) Ok
   printSomething();        // (2) Does nothing

   Notes

8. A function that overwrite itself is also called a self-defining function.

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