Final answer:
We use the dot operator (.) or square brackets ([]) to access properties of an object returned by document.getElementById, depending on the property name's validity as an identifier and if the name is known at runtime.
Step-by-step explanation:
The operator we use to access different properties of the object returned by document.getElementById is the dot operator (.), or sometimes the square brackets ([ ]) depending on the context and the property you are attempting to access.
For example, let's say you have an HTML element with the id 'myElement' and you want to change its text content. You can access the textContent property of the object by using:
var myElement = document.getElementById('myElement');
myElement.textContent = 'New text content';
If you wanted to access a property that has a name which is not a valid identifier (for example, a property name that includes spaces or hyphens) or is not known until runtime, you might use the square brackets notation:
myElement['data-custom-attribute'] = 'value';