In what manner does the Date class's constructor modify the unary + operator?

While browsing through this answer regarding the Hidden Features of JavaScript?, I came across a perplexing behavior:

> new Date
< Fri Feb 26 2016 21:15:43 GMT-0500 (EST)
> +new Date
< 1456539382581
< 0 + new Date + 0
"0Fri Feb 26 2016 21:17:39 GMT-0500 (EST)0"

I was astonished to discover that the Date constructor returns an object which, when converted to a number via unary operation (but not addition), yields a number representing the Unix Epoch.

This raises the question: How is this possible? After researching the topic, it appears that JavaScript lacks support for operator overloading, or only offers limited overloading of non-operator functions like add and toString.

Considering JS doesn't appear to have a specific function for unary + (or any number coercion mechanism at all), how exactly does this functionality work?

javascriptoperator-overloading

Answer №1

Using the unary operator + will convert its operand to a number if it is not already one.

For example, +new Date is essentially the same as Number(new Date).

new Date() // Fri Feb 26 2016 21:45:32 GMT-0500 (EST)

Number(new Date()) // 1456541132899

+new Date() // 1456541132899

Update:

What makes calling Number on a Date object return Unix time?

Date objects are based on a time value representing milliseconds since January 1, 1970 UTC (according to MDN). Converting it to a number results in milliseconds being returned.

Answer №2

Understanding the Conversion of Dates in JavaScript

When working with Date objects in JavaScript, it's important to know how automatic type conversion takes place. The primitive numeric value of a Date object represents a Unix timestamp, which is the number of milliseconds since January 1, 1970 UTC.

One method of converting a Date object to a number is by using the unary + operator. This operation calls the object's internal toPrimitive method with a hint value of "number", resulting in the conversion to milliseconds.

However, when a Date object is used as the operand of a non-unary plus, it gets converted to a string instead of a number. This is because the conversion process does not provide a result type hint, leading to the default behavior of converting dates to strings using the exotic toPrimitive function.

Answer №3

Explained below is the concept of unary plus as described in MDN:

The unary plus operator comes before its operand and yields the same value, but it tries to convert it into a number if it's not already one.

In case the operator cannot interpret a given string or value, it results in NaN.

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