JavaScript Particularities that Will Throw You Off - Part 1

I recently started looking into JavaScript after having learned Java as my first programming language and oh boy... there are many weird things happening under the hood in JavaScript. This article will be a list of such oddities I notice while continuing to learn JavaScript. A continuation may follow in later blog posts.

Dynamic typisation

Javascript is unlike Java dynamically typed, there is therefore no need to declare an explicit type for methods and even for most data structures, from what I gather. There is also no need to declare the return type.

Automatic type conversion

Type conversion is not a new feature of programming languages, but JavaScript works again differently. While JS converts the integer 1 in the case of addition to a string data type, it does the completely opposite when substraction is involved - JS converts the string to an integer:

console.log("5" + 1);
// ->51
console.log("5" - 1);
// ->4

Using == or != also induce automatic type conversion:

console.log(null == undefined);
// -> true

console.log(0 == false);
// ->true

console.log("" == false);
// ->true

To be sure that no automatic type conversion happens, always use "===" and "!==":

console.log(null === undefined);
// -> false

console.log(0 === false);
// -> false

console.log("" == false);
// -> false

Logical operators and funny behavior

Logical operators such as "OR" ( || ) and "AND" ( && ) will only operate the part to their right when necessary:

console.log(null || "user")
// -> user
console.log("Caesar" || "user")
// -> Caesar
console.log("false && X")
// -> false

Pay therefore attention with JS ;).