Why Does JavaScript\'s Ternary Operator Behave Differently When Used with the ` =` Operator?

JavaScript's Ternary Operator and Operator Precedence

Understanding the order in which operations are performed in code is crucial to avoid unexpected results. In JavaScript, the ternary operator presents a common challenge, especially when combined with other operators.

Consider the following code snippet:

h.className += h.className ? ' error' : 'error'

One might assume that the code operates as follows:

h.className = h.className + h.className ? ' error' : 'error'

However, this interpretation is incorrect as it results in a console error. To correctly understand the code, it's essential to consider operator precedence.

Answer:

The key to understanding the code is to realize that the operator takes precedence over the ternary operator. Therefore, the code evaluates as follows:

h.className = h.className + (h.className ? ' error' : 'error')

In this case, the operator concatenates h.className with the result of the ternary operator. The ternary operator checks the truthiness of h.className and returns ' error' if it's true, or 'error' otherwise.

To avoid confusion, it's advisable to be explicit about the target of the operation. In this case, using parentheses to group the expression ensures that the operator is applied to h.className, as intended.

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