Decoding the Enigmatic =_ Operator in JavaScript

The uncommon operator =_ in JavaScript has perplexed developers, leaving them wondering about its true nature. This operator combines the assignment operator = with the unary plus operator _. Let's delve into its intricacies and uncover its purpose.

Unary Plus Operator ( _)

The unary plus operator ( ) is a prefix operator that attempts to convert its operand into a number. It performs the following tasks:

• Converts string representations of integers and floats to numbers
• Converts non-string values like true, false, and null to numbers
• Supports integers in decimal and hexadecimal formats
• Evaluates to NaN if it encounters a value it cannot parse

Code Example:

 "1"; // converts "1" to the number 1

Assigning a Parsed Value:

The =_ operator combines the above conversion behavior with assignment. For instance, in the code below:

hexbin.radius = function(_) {
    if (!arguments.length)
        return r;
    r =  _;
    ...
};

The _ variable acts as a placeholder for the argument passed to the function. The unary plus operator ( ) attempts to convert the argument to a number and assigns the result to the r variable.

Example:

var _ = "1";
var r =  _;

After execution, r will contain the number 1, not the string "1." This conversion is significant in many scenarios, such as mathematical calculations and data handling, where numerical values are essential.

Advantages of _:

According to the MDN page on Arithmetic Operators, the unary plus operator is the "fastest and preferred way of converting something into a number." This efficiency makes it an ideal choice for situations where performance is critical.