JavaScript data types and data structures

JavaScript data types and data structures
Programming languages all have built-in data structures, but these often differ from one language to another. This article attempts to list the built-in data structures available in JavaScript and what properties they have. These can be used to build other data structures.

Dynamic and weak typing
JavaScript is a dynamic language with dynamic types. Variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned (and re-assigned) values of all types:
let foo = 42; // foo is now a number
foo = "bar"; // foo is now a string
foo = true; // foo is now a boolean


JavaScript is also a weakly typed language, which means it allows implicit type conversion when an operation involves mismatched types, instead of throwing type errors.
const foo = 42; // foo is a number
const result = foo + "1"; // JavaScript coerces foo to a string, so it can be concatenated with the other operand

Implicit coercion is very convenient but can be a potential foot gun if developers didn't intend to do the conversion, or intend to convert in the other direction (for example, a string to a number instead of a number to string). For symbols and BigInts, JavaScript has intentionally disallowed certain implicit type conversions.