Salesforce JavaScript Developer Practice Exam

What is the output when logging a[b] after a and b are modified?

1. 123

2. 456

3. undefined

4. ReferenceError

The correct answer is: 456

When logging `a[b]` after the modifications to `a` and `b`, seeing the output as 456 indicates that `b` was set to a valid key within the `a` object, and that this key corresponds to the value 456. In JavaScript, objects are collections of key-value pairs, and the value can be accessed using either dot notation or bracket notation. If `b` holds the string value that matches a key in the object `a`, then `a[b]` will yield the corresponding value. Therefore, if `a` is modified such that a property with `b` as the key exists and has a value of 456, logging `a[b]` will result in this output. This signifies that both `a` and `b` were appropriately set up prior to the logging statement, where the modification consisted of adding a property to `a` with the key specified by `b`, and assigning a value of 456 to it. Hence, when `a[b]` is evaluated, it retrieves that value.