Now you might ask – what about ESLint? It’s used in the modern projects and it enforces triple-equals. We can take advantage of the double-equals and the fact, that (null = undefined) is true. So what should we do to avoid the tedious (val = null || val = undefined) check? In such a scenario, the expression will resolve to true, even though value isn’t null or undefined (which is what we probably intended to check in the first place). But what if val is a nullable numeric value?

It’s correct, but it’s also long, so people very often shorten it to And here comes the trap. Very often we have to check if the value is null or undefined before we perform any further operations. Now, let’s see the examples that explain the above. It checks if two values are the same. Unlike double equals, Object.is doesn’t coerce either value. It differs from triple equals with the way it treats signed zeros and NaNs. Object.is provides SameValue (new in ES2015).It doesn’t convert the types before performing the comparison, which means that it’s type-sensitive. = Strict Equality Comparison (“strict equality”, “identity”, “triple equals”).Before checking the values. it converts the types to match each other.

= Abstract Equality Comparison (“loose equality”, “double equals”).

There are three value-comparisons in operations in JavaScript: Let’s start with a little theory, and later I’ll guide you through examples and finally show you a painless (and safe!) way to do the null checks in the code. In JavaScript, you will encounter null or undefined. To make life even harder, there are also double (abstract =) and triple (strict =) equality comparisons which combined with null/undefined confusion can be a challenge, especially for beginners. In most strongly typed languages the value can be either null or not, however that’s unfortunately not the case in JavaScript. Checking if the value is null or undefined in JavaScript (and TypeScript) can be a real headache and doing it incorrectly can lead to unexpected errors during runtime.

Javascript null check