4.1 Plain Objects

>>>

var obj = { a: 'value', b: 123, c: null }

Object instances are basically ordered sets of named values (properties). See 1.2 Object Types for some common built-in object types.

>>>

Object.keys(obj)

["a", "b", "c"]

Property names can be listed with the built-in Object.keys() function. See also 2.6 Loops & Iteration for iteration over object properties.

>>>

obj.a

"value"

Property access is an O(1) (constant time) operation, making any object a candidate lookup table. See 2.2 Property Access for details.

>>>

obj.toString

function toString() {
    [native code]
}

All objects also have some built-in non-enumerable properties, such as toString, valueOf and hasOwnProperty. These typically contain functions that operate on the object, also known as methods. See the Object instance properties for a compete list.

>>>

obj.toString()

"[object Object]"

The built-in toString() method works for all objects. But the default implementation isn't all that exciting.

>>>

{ a: 1 } === { a: 1 }

false

Also worth noting is that object instances are never equal unless it is actually the same instance.

>>>

The console allows you to interact with the course material and examples. Use the following keys:

<Enter> — Execute the code
<Shift + Enter> — Add line break
<Arrow-Up> — Previous command in history
<Arrow-Down> — Next command in history

A special logging function is also available:

print(value) — Prints a value to the console.
print(value, true) — Prints a debug version of a value.

Console Learner — An interactive JavaScript course.