What does the Plus Symbol ( ) Do When Placed Before a Variable in JavaScript?

What is the Role of the Plus Symbol Preceding a Variable?

In programming languages, the plus symbol ( ) is commonly used with numeric data types to perform addition operations. However, it can also be placed before variables to serve a different purpose.

Specific Context: The d in addMonths Function

Let's consider the following code snippet from a JavaScript function called addMonths:

function addMonths(d, n, keepTime) { 
    if (+d) {

Here, the variable d is preceded by a plus symbol. What does this mean?

Purpose of d: Checking for a Non-Zero Numeric Value

In this context, the operator returns the numeric representation of the object. So, d effectively converts the variable d to a number.

The if statement checks whether the result of d is true, which in JavaScript is the same as checking whether it is a non-zero numeric value. If d is a number other than 0, the condition is true and the code block within the if statement will execute.

Understanding the Numeric Representation

When applied to a variable, the operator attempts to convert the variable into a numeric value. If the variable is already a number, it remains unchanged. Otherwise, the variable is coerced into a number using the following rules:

• Strings are converted to numbers, interpreting them as decimal values.
• Booleans are converted to 0 (false) or 1 (true).
• Other data types may be treated as NaN (Not-a-Number).

Example

If d is a string containing the value "15", d will return the number 15. Consequently, the if statement will be true and the code block will execute.

Reference

• [Numeric Conversion in JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unary_plus)

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