At Mastering JS, we think learning JavaScript fundamentals is much higher ROI than learning any particular framework. Two reasons why:

1) Fundamentals rarely change. The JavaScript language spec doesn't allow backwards breaking changes. The void operator will work the same way in 5 years as it does today.
2) Fundamentals are the same regardless of what framework you use. If you switch jobs tomorrow, you'll still be able to apply your fundamental language skills even if your new job does things differently.

7. The Promise then() Function in JavaScript

Promises and promise chaining, explained.

6. Understanding Array.splice() in JavaScript

The splice() function lets you add and remove elements from the middle of an array. Here's how it works.

5. Compare Dates in JavaScript

Checking whether two dates are equal in JavaScript is tricky, this tutorial explains how. On the bright side, you can use < and > to compare whether one date is before or after another.

4. What is a Plain Old JavaScript Object (POJO)?

Whether an object is a POJO or not is a nuanced and commonly debated topic. This tutorial goes into detail about why checking whether a value is a POJO is tricky.

3. Iterating Through an Object with forEach()

Some ESLint presets require you to use forEach() rather than conventional loops. That means you need to iterate through objects using forEach(). This tutorial shows you how.

2. [How to Use forEach() in JavaScript

](https://masteringjs.io/tutorials/fundamentals/foreach)

While we do not recommend using forEach() to iterate arrays in JavaScript, forEach() is an established part of JavaScript and worth mastering. This tutorial tells you what you need to know about forEach().

1. 3 Ways to Concatenate Strings in JavaScript

Concatenating two strings is one of the most fundamental tasks in JavaScript. This tutorial explains the tradeoffs of 3 different ways to concatenate strings.