Coding is hard, that's why not a lot of people do it. So I can assure you that this, you struggling, is going to happen again. You have to be ok with not fully understanding everything, mainly because you don't need to, to be a successful programmer. More on reddit.com

Some differences include: for loops for (let i = 0; i < arr.length; i++) { // loop code } User defined loop conditions Allows breaking (and continuing) Supports await/yield for surrounding scope Standard usage for arrays gives you index only, not value Tends to be more complicated than other loops (less readable, more error prone) But also most flexible in how looping is defined for of loops for (const element of arr) { // loop code } Loops through iterables only Allows breaking (and continuing) Supports await/yield for surrounding scope Standard usage for arrays gives you value only, not index Other helpers needed for specific kinds of iteration, like array.entries() to get indices and values, or Object.keys() for going through keys of objects For arrays, loops through all values, even holes when arrays are sparse for in loops for (const key in obj) { // loop code } Loops through any value Iterates through value keys, both own and inherited (Object.keys(), by comparison, only provides own keys, not inherited) Allows breaking (and continuing) Supports await/yield for surrounding scope For arrays does not include holes when arrays are sparse (not recommended for looping through arrays) forEach loops arr.forEach(function (element, index, arr) { // loop code }) Only usable for collections that have this method (arrays, typed arrays, and some others like NodeLists from DOM API) Uses a callback function rather than looping through a code block Does not allow breaking Does not support await/yield for surrounding scope (though you can await and yield in the callback itself, it doesn't affect the looping) Gives you value, index, and collection being looped for each iteration For arrays does not include holes when arrays are sparse Often considered the most readable, especially when reusing callback functions (arr.forEach(doThing)) though can also be least performant More on reddit.com

You can use the "break" keyword in for, you can't do this in foreach. So for and foreach exist for different use case. More on reddit.com

You're trying to see key as a parameter. But it's not a parameter, it's a variable declaration, much like i in your first for loop. You don't have to give it an initial value or increment it or anything though, the runtime does that for you as it iterates over object's properties; all you have to do is consume key. More on reddit.com