When are parentheses and new keywords required when creating new objects in JavaScript?

Question

I'm studying objects in JavaScript and confused when the new keyword and () are required when using a object creation expression.

var a = new Date();     // current date and time
var b = Date();         // current date and time
var c = new Date;       // current date and time 
var d = Date;           // => function Date() { [native code] }

Is there any difference in the first three methods? Why doesn't d do as expected?

Answer 1

Given: new Date()

This is the "standard" way to create a new object from a constructor-function¹; it returns a new Date object representing the current time.

Given: Date()

JavaScript defines the Date function to operate like this when not called as a constructor (ie. with new). It returns a different value - a string and not a Date object - than new Date(). See Why we can't call methods of Date() class without new operator.

User-code can also check the value of this inside a constructor-function to determine if new was used, although this is only done relatively infrequently. See How to detect if a function is called as constructor?

Given: new Date

When using new the parenthesis are optional if there are no arguments. This is just an additional syntax form that most people don't use - it is equivalent to new Date() if the expression is terminated. See Can we omit parentheses when creating an object using the "new" operator?

Given: Date

This evaluates the expression to the constructor-function (which is just a function-object!²) without invoking it or creating a new instance. This is why the result shows a "function" - it is the constructor-function.

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¹ I use the term constructor-function to emphasis the point that a constructor is only a function that [also] supports new; it is technically sufficient to call it a constructor.

² Likewise, it is important to understand that function[-objects]s in JavaScript are first-class citizens and thus just values themselves; like any object. In the last example the assignment of the object/value occurred without performing any action (ie. function invocation) upon the object itself.