Why does `${\'h.className = h.className ? \' error\' : \'error\'}` not work as expected in JavaScript?

Troubleshooting Ternary Operator Precedence in JavaScript

JavaScript's ternary operator is a powerful tool for conditional assignments, but understanding its precedence can be tricky. This article focuses on a specific scenario where the ternary operator is combined with =.

Understanding the Problem

Consider the following code snippet:

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

At first glance, the code appears to concatenate the current value of h.className with either ' error' or 'error' based on the ternary condition. However, this interpretation can lead to errors.

Correct Interpretation

The issue lies in the precedence of the operators. In JavaScript, the operator has higher precedence than the ternary operator. This means that the expression above is evaluated as follows:

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

Solution

To ensure that the ternary operator is applied correctly, the code should be written as:

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

This ensures that the ternary operator is evaluated before the concatenation with h.className.

Additional Considerations

It's important to note that h.className = ' error' is also valid, but it's not as precise as the updated code. The = operator applies only to h.className, whereas the updated code explicitly concatenates the ternary condition result with h.className.

Conclusion

Understanding operator precedence is crucial for writing correct JavaScript code. By clarifying the precedence rules for the ternary operator and the = operator, this article provides a solution to a common challenge encountered when using conditional assignments in JavaScript.

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