String concatenation

You can add (+) two strings this produces a new string with both inputs concatenated:

const a = "full";
const b = "stack";
console.log(a + b); // --> fullstack

One problem with + for strings is that it will auto-convert arguments into strings if it needs to, which can be confusing:

1 + "2"; // --> "12" (not 3!)
"5" + 5; // --> "55" (not 10!)
2 + 3 + "4"; // --> ???

Typical Operators

The operators which come from C, C++, Java, etc. are all supported and mean the same:

Modify in place

Other operators from C include the assignment operators:

Javascript is weird

The problem mentioned in string concatenation is in fact widespread. To be "nice" to users and not produce any errors, the original Javascript would make any malformed expression valid by auto-converting its arguments to other types implicitly:

true +
  false[(1, 2, 3)] + // --> ??
  [4, 5, 6]; // --> ??
10, 2; // --> ??
!!"" + // --> ??
  !![]; // --> ??

This Javascript weirdness is frowned upon by many programmers. The trick is to avoid the worst of these expressions.

Triple equality

The most affected operator is ==, equality (and !=, inequality). Since auto-conversion will kick in before comparison, in Javascript all these expressions give weird values:

0 == "0"          // --> true
0 == null         // --> true
NaN != NaN        // --> true
null == undefined // --> true
{} == {}          // --> false (this one is a trap)

Since Javascript could not change the comparison operators, what they did in ES5 was to introduce two new operators: === and !==. These fix the old operators and therefore you should always use them. These operators always check the type first, so obviously false comparisons are always easy to reason about.

Not a Number equality

There is one missing case, though:

NaN === NaN; // --> false

To check if a number is "not a number", then, you cannot write:

if (a === NaN) {
  console.log(`${a} is NaN!`);
}

A special function Number.isNaN does the checking right:

if (Number.isNaN(a)) {
  console.log(`${a} is NaN!`);
}