a["b"]["d"]

You can use the array notation for both properties and methods. The general syntax is:

objectReference["propertyName"]
objectReference["methodName"]([arguments])

It is important to understand the array notation because sometimes it is the only alternative. For example, you cannot use the dot notation when the first character of a property is a digit. When using the array notation, the name of the property must be enclosed in quotes. Here are some examples:

document["write"]("hello")
window["alert"]("howdy!")
Math["PI"]

You must use the array notation when you need to reference the name of the property through a variable. Suppose the variable str holds the string "write". Instead of document.write(), you can use:

document[str]()

However, you cannot use document.str() because it is equivalent to document["str"]().

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