JavaScript Type System
8 lessonsAbout 48 minutesIntermediate
This part of the JavaScript Tutorial runs from JavaScript typeof through to JavaScript Primitives and References. There are 8 lessons here, and working through them takes about 48 minutes at a steady pace.
It follows on from JavaScript Classes, so finish that first if you have not already — the examples below assume you are comfortable with it.
Each lesson below says what it covers before you open it. Read them in order the first time; afterwards this page works as a index you can jump back into when you need to check one thing.
- 1JavaScript typeofThe typeof operator reports what kind of value you are holding.
- 2JavaScript Type ConversionType conversion changes a value from one type to another on purpose.
- 3JavaScript Type CoercionType coercion is JavaScript converting types automatically, whether you wanted it or not.
- 4JavaScript == vs ===The double equals converts types before comparing; the triple equals does not.
- 5JavaScript Truthy and FalsyEvery JavaScript value is either truthy or falsy when used as a condition.
- 6JavaScript null and undefinedundefined means a value was never set; null means it was set to nothing on purpose.
- 7JavaScript NaNNaN means Not a Number, and it is the result of a failed number operation.
- 8JavaScript Primitives and ReferencesPrimitives are copied by value and objects are copied by reference.