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Data types

JavaScript includes 8 data types.

  • Seven are primitive data types1

    • undefined: a special value automatically assigned to identifiers if they don't have any value
    • null: A keyword denoting a nonexistent value, but implicitly assigned
    • boolean: contains true and false values
    • string: a sequence of characters that represents a text value
    • number: an integer or floating point number
    • bigint: an integer with infinite precision
    • symbol: a data type whose instances are always unique

  • And one is a non-primitive, or composite, data type

    • Object
Note

typeof null will return object instead of null. This behaviour has some historical reasons.

Note

-Infinity, +Infinity and even NaN (not a number) are a special values of the number data type. And NaN it's also the only value in JavaScript that isn't equal to itself when we use the loose or strict equality operator to compare it (only Object.is(NaN, NaN) will return true).

Primitives​

Immutability2​

It is important not to confuse a primitive value itself with a variable assigned a primitive value. Primitives are immutable, so if you intend to alter the variable containing a primitive, you need to replace its value with a new one.

Strings are immutable, so you can't replace one letter in the string value. Symbols are immutable also.

Auto-boxing​

Technically, primitives have no methods or properties, but they behave as if they do. When we try to access property or method of a primitive, JavaScript temporary converts the primitive to the corresponding object.

Objects​

Objects are stored in memory, and variables reference them by links. In JavaScript, objects are the sole mutable values.

Arrays, Dates, Sets, Maps, and many more structures in JavaScript are also objects. Additionally, functions are objects, with the added capability of being callable.

Each object represents a collection of key-value pairs. Keys are either strings or symbols, and values can be of any data type, including other objects.3

Type converson​

JavaScript is a weakly typed language, so values can be converted to another type when needed. This called impicit type conversion or type coercion. For example, when we use logical negation operator ! followed value is coerced to boolean. Invoking console.log() with any non-string argument triggers string coercion.

Values can be converted different ways: to primitives (it’s called Primitive coercion), to strings (String coercion), to booleans (Boolean coercion), to numbers (Numeric coercion) and to object (Object coercion). However, values cannot be coerced to null, undefined, or symbol. In other words, these data types don't have their coercion process.4

Manual transformations of data from one type to another called explicit type conversion and achieved through functions like parseInt(), Object() or methods like .toString.

Footnotes​

  1. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#data_structures_and_types ↩

  2. https://developer.mozilla.org/en-US/docs/Glossary/Mutable ↩

  3. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#objects ↩

  4. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#type_coercion ↩