Congratulations! You are wading into "higher-order functions" (i.e. functions that call other functions). This is a powerful technique which underpins a lot of super useful JavaScript methods, but is often confusing at first. Best thing you can do is open up your console (or wherever you prefer to run JavaScript code), and mess around until it starts to click. Let's look at some examples you can start with. Let's clear up your question first. No, functions are not called when you declare them. Like in PHP and other languages, declaring a function is essentially just stashing a code snippet away for later. Nothing at all will happen with that code until the function is called. We can experiment with this ourselves by defining the JS equivalent of that PHP function you posted and not calling it. function helloWorld() { console.log("Hello world!"); } . . . Nothing logged! Similarly, nothing will log if we define the equivalent function with the arrow syntax but do not call it. const helloArrow = () => console.log("Hello arrow!"); Don't take my word for it! I encourage you to open your console and copy and paste this code in. See for yourself that nothing happens. And make any modifications you want if you are curious about some edge case or something. Okay, now just to make sure we're not crazy, let's call these functions. helloWorld(); // Hello world! helloArrow(); // Hello arrow! helloWorld(); // Hello world! helloWorld(); // Hello world! All appears to be in order. Let's actually move on to higher-order functions. So, your confusion around map stems from the function getting invoked "without anyone calling it". But in fact, someone did call your arrow function. You did. Think of it like a Rube Goldberg machine. You called map and passed it your function. Then map calls your function. Multiple times in fact. Once per item in the array. Let's try the same thing with another higher order function, forEach . What will happen if we pass it our helloWord function? const nums = [1, 2, 3]; nums.forEach(helloWorld); // ??? Make your own hypothesis, then copy that code into your console, run it and find out. See if your hypothesis matches reality. If not, what were you missing? Answer below: "Hello world!" gets logged to the console three times. Okay. So let's bring it back to map. I think the best thing we can do is build our own version, so you can see the mechanics of the Rube Goldberg machine for yourself. I'm going to build a slightly simplified version of JS's map using a for...of loop . const myMap = (arr, fn) => { const mapped = []; for (const item of arr) { mapped.push(fn(item)); } return mapped; }; Since this is a function not a method, we will call it a little differently than JS's version, the array will go in between the parentheses instead of to the left of the dot. But the result will be the same. const doubled = myMap(nums, x => x * 2); console.log(doubled); // [2, 4, 6]; So who is calling the function we passed into myMap? Well, myMap is. Now that we've written our own version, you can see it happen above, right in the middle of the for loop. The point of map and other higher-order functions is to handle the mechanics of a generic operation you need to do often, like mapping over the data of one array in order to transform it into a new array. You get to skip all that boilerplate. Just define your particular business logic (like doubling a number), and pass it in as a function. Once again, I can't emphasize enough how useful it is to copy and paste this code into your console so you can play around with it yourself. Do that for awhile, and if you have any more questions, feel free to ask. More on reddit.com

So you might consider instantiation ... could also be considered an invocation rather than a call. But W3Schools' insistence on using "invoke" everywhere seems very unnatural to me. ... "because a JavaScript function can be invoked without being called"...... More on stackoverflow.com

Is there a way in Javascript to define a function and immediately call it, in a way that allows it to be reused? I know you can do one-off anonymous functions: (function(i) { var product = i... More on stackoverflow.com

This is fine. They are called nested functions. As long as you keep things organized and it's not becoming spaghetti code, there is no problem. Just remember that variables you create witin a function will stay local. And actually, this is a good way to save time. Any kind of operation you need repeated regularly, you should create a function for it; reduces amount of code. More on reddit.com