Functions
Functions
In many programming languages, there exists a concept called a function. In JavaScript, like many others, a function is one or more lines of code usually, but not always, written for some task or purpose.
The purpose of functions is to logically separate sections of code. This allows for re-using code and helps to organize things.
The terminology around function usage is that functions are called and that they may return a value.
Functions
In JavaScript, like with many other programming languages, there are some different common parts of usage to functions. The first is the name.
To use some code as a function, it must be defined as a function. This means that there is a section with the keyword “function.” After this, is its name. When a function is used, it called through its name.
Functions, similar to variables, are also defined. They have definitions. This includes all code with the opening and closing curly brackets.
The third part is its parameters. These are things sent to the function. To help abstract functions as sections of code, they can accept values. Within the function these parameters are defined using the names given to them.
Finally, there is the return statement. If the function does some task and needs to send a value back, it can do so using the “return” keyword. Anything returned by the function can then be saved and used for another task.
Calling Functions
When using a function in JavaScript, it is called. This means that control is temporarily moved to that section of code, the function does something, and then control is usually returned back.
Depending on what is sent as a parameter, the function can use those values or not. Then, upon coming to a return statement or the end of the code, control moves back.
console.log() that you have already seen is, in fact, a function call. The function log() is called and the parameters of different values were passed to it to show the values of different variables and even locations in an array.
Functions as Values
Like other types of values, functions themselves can also be saved to a variable. When doing this, the way the function is called changes. Now, instead of its original name, the name of the variable can be used in its placed to call that section of code.
When would you do this? Well, normal functions are created at the instant the code is run. Once a JavaScript is executed, all functions are "hoisted" to the top of the code and created immediately.
However, this doesn't happen when you create a function inside of a variable. This type of function is created only when the browser reaches the line of code where the variable is set.
If you were making many functions the normal way (without variables), the code will read and create all of those functions before any code can be run. This could slow down the non-function JS, which might not be necessary, since you probably don't need to run all of the functions immediately. If you use variables, you can set functions just before they're needed, and avoid that initial slowdown. The key is that var functions must be set in an earlier line of code than the line they are used. Non var functions can be used anywhere, since the function is created as soon as the code is run.
Anonymous Functions
Because saving a function as the value of some variable removes its original name, functions can also be anonymous. When a function is defined as anonymous, it does not have a name. However, as with setting a variable with the value of a function, it can still be used. The variable allows for calling the function based on its name, instead.
Values from Functions
The values returned from functions can also be used as part of the definition of other variables and as part of other tasks. Because the call to the function can return a value, that value will be substituted later. It can represent a potential value that will be calculated when the code is run.
You might use this in a case where you want to actions on some data to think about the relationships it has to other data, such as looking as social media engagement as numbers and as ratios.
Function Scope
JavaScript shares a concept with many other programming languages called scope or sometimes function scope. This means that variables defined and enclosed within a function cannot be used outside of it.
In fact, trying to use or reference a variable outside of its scope will cause an error and usually stop the code from running.
Local and Global Scope
There are two types of scope: local and global. When a variable or function is defined within a function, it is said to be part of the local scope. They can be used within the scope of that function.
Variables and functions defined outside of functions are part of the global scope. This means they can be used inside of other functions because they are defined as part of a larger scope that contains the function in which they are defined.
A handful of keywords are part of the highest level of scope and can be used anywhere in other functions when JavaScript is run inside of a web browser. They include the variables window, document, and console.
One thing that you might use this for is to count up both individual social media likes and those of all users:
Play with the Code (Repl.it):
Take a moment to play with the code and concepts in the embedded Repl.it viewer below. You might try changing the arithmetic (+, -, *, or /) or the numbers in the equation to see how it changes the outcome.