February 2015
Beginner
198 pages
3h 45m
English
Content preview from You Don't Know JS: Types & Grammar
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Chapter 1. Types
Most developers would say that a dynamic language (like JS) does not have types. Let’s see what the ES5.1 specification has to say on the topic:
Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further sub-classified into ECMAScript language types and specification types.
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.
Now, if you’re a fan of strongly typed (statically typed) languages, you may object to this usage of the word “type.” In those languages, “type” means a whole lot more than it does here in JS.
Some people say JS shouldn’t claim to have “types,” and they should instead be called “tags” or perhaps “subtypes.”
Bah! We’re going to use this rough definition (the same one that seems to drive the wording of the spec): a type is an intrinsic, built-in set of characteristics that uniquely identifies the behavior of a particular value and distinguishes it from other values, both to the engine and to the developer.
In other words, if both the engine and the developer treat value 42
(the number) differently than they treat value "42" (the string), then
those two values have different types—number and string,
respectively. When you use 42, you are intending to do ...
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