Functions act as placeholders for the data they output. Basically, you can assign the output of a function to a variable, just like any normal data.The core concept of this lesson is to learn how to call a function and assign the return value to a variable. One of the things I wanted to change was to turn the assignment operator into an assignment command, removing the ability to make use of the returned value. FreeCodeCamp offers a curriculum and in this video we cover their lesson called Assignment with a Returned Value. Take a hidden field or Label in the form. Assign return value to the Hidden form with document.getElementById('ID'). We can take the return value of a function and assign it to a variable. Assume we have pre-defined a function sum which adds two numbers together. You are passing the vector as const and you are trying to modifying it.