Speaking JavaScript

Chapter 8. Values

JavaScript has most of the values that we have come to expect from programming languages: booleans, numbers, strings, arrays, and so on. All normal values in JavaScript have properties.^[9] Each property has a key (or name) and a value. You can think of properties like fields of a record. You use the dot (.) operator to access properties:

> var obj = {}; // create an empty object
> obj.foo = 123;  // write property
123
> obj.foo  // read property
123
> 'abc'.toUpperCase()  // call method
'ABC'

JavaScript’s Type System

This chapter gives an overview of JavaScript’s type system.

JavaScript’s Types

JavaScript has only six types, according to Chapter 8 of the ECMAScript language specification:

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are:

• Undefined, Null
• Boolean, String, Number, and
• Object

Therefore, constructors technically don’t introduce new types, even though they are said to have instances.

Static Versus Dynamic

In the context of language semantics and type systems, static usually means “at compile time” or “without running a program,” while dynamic means “at runtime.”

Static Typing Versus Dynamic Typing

In a statically typed language, variables, parameters, and members of objects (JavaScript calls them properties) have types that the compiler knows at compile time. The compiler can use that information to perform type checks and to optimize the ...