Callbacks

• Define a callback function

• Demonstrate ability to pass a function as a parameter

Functions may seem very different than the other javascript items we've worked with before, but don't be fooled! Functions are still first-class values, which means you can

• store them in variables

• pass them as arguments to other functions

• create them within functions

• return them from functions

If this all seems very strange, you could think of a javascript function as a javascript object that stores code to be run instead of key/value pairs. A callback function is just a regular function that we pass as an argument to another function!

Take a look at this example:

const greeting = () => {
    console.log("Hello, World!");
}

const formalGreeting = (informalGreeting) => {
    informalGreeting()
    console.log("How are you?")
}

formalGreeting(greeting)

When we pass a function into another function as a paramenter, we are passing the function definition, so it can be called at a later time (hence the name callback). Notice the different outputs in the below example. First we print the function itself, then we call the function and print the output.

const returnRandom = () => {
    return (Math.random()*100).toFixed();
}

const yellRandom = (randomNumGenerator) => {
    console.log("GENERATOR DEFINITION: ", randomNumGenerator)
    console.log("YOUR RANDOM NUMBER IS: "+randomNumGenerator());
}

yellRandom(returnRandom)

Anonymous Functions

If you define a function that is only going to be used as a callback, and not called anywhere else, you can pass skip the functioning declaration and just define it as an anonymous function within the encompassing function call itself. An anonymous function is a function that isn't stored in a variable.

const formalGreeting = (informalGreeting) => {
    informalGreeting()
    console.log("How are you?")
}

formalGreeting(()=>{
    console.log("Hello, World!");
})

Exercise: call testTypes and pass in a anonymous function that takes one argument and prints that argument using console.log.

const testTypes = (someFunc) => {
    console.log("num input:")
    someFunc(0)
    console.log("bool input:")
    someFunc(true)
    console.log("str input:")
    someFunc('some string')
    console.log("numInput:")
    someFunc([1,2,'3'])
}