4.7 Native Value Objects

>>>

new Number(1)

As mentioned in 1.2. Object Types, JavaScript contains a number of built-in object types (or constructor functions, really).

>>>

new Number(1) == 1

true

It is common to make assumptions about these objects...

>>>

new Number(1) == new Number(1)

false

...and be wrong. In reality, the Number objects are wrappers around a primitive number value. They are not one and the same.

>>>

new Number(1).valueOf()

The built-in valueOf() method helps to explain what just happened. It is called to coerce an object to a primitive value type (i.e. boolean, number or string).

>>>

new Number(1) === 1

false

When avoiding type coercion, the first comparison from above now shows that the values are indeed different. The second result from above is explained by normal object comparison.

>>>

(1).constructor

function Number() {
    [native code]
}

But lets not pretend that logic always prevail. Any primitive boolean, number or string value will happily masquerade as an instance of the corresponding wrapper object.

>>>

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.