Javascript: Identifying Primitive and Reference Types

Identifying Primitive Types

There are five primitive types in JavaScript:

• boolean
• number
• string
• null
• undefined

The best way to identify primitive types is with the typeof operator, which works on any variable and returns a string indicating the type of data.

console.log(typeof "John Doe");   // "string"
console.log(typeof 10);           // "number"
console.log(typeof 5.1);          // "number"
console.log(typeof true);         // "boolean"
console.log(typeof undefined);    // "undefined"

When you run typeof null, the result is "object". The best way to determine if a value is null is to compare it against null directly, like below. Make sure that when you’re trying to identify null, use triple equals so that you can correctly identify the type.

console.log(value === null);    // true or false

Identifying Reference Types

The built-in reference types are

• Array
• Date
• Error
• Function
• Object
• RegExp

A function is the easiest reference type to identify because when you use the typeof operator on a function, the operator should return "function":

function reflect(value) {
 return value;
}
console.log(typeof reflect);   // "function"

Other reference types are trickier to identify because, for all reference types other than functions, typeof returns "object". To identify reference types more easily, you can use JavaScript’s instanceof operator. The instanceof operator takes an object and a constructor as parameters. When the value is an instance of the type that the constructor specifies, instanceof returns true; otherwise, it returns false.

var items = [];
var object = {};

function reflect(value) {
 return value;
}
console.log(items instanceof Array);    // true
console.log(object instanceof Object);    // true
console.log(reflect instanceof Function);   // true

Array.isArray() is the best way to identify arrays.

var items = [];
console.log(Array.isArray(items));    // true