Question

We’ve seen a few suspicious-looking expressions like myString.length (to get the length of a string) and Math.max (the maximum function) in past examples. These are expressions that access a property of some value. In the first case, we access the length property of the value in myString . In the second, we access the property named max in the Math object (which is a collection of mathematics-related values and functions).

Almost all JavaScript values have properties. The exceptions are null and undefined . If you try to access a property on one of these nonvalues, you get an error.

null.length;
// → TypeError: Cannot read property 'length' of null

The two most common ways to access properties in JavaScript are with a dot and with square brackets. Both value.x and value[x] access a property on value —but not necessarily the same property. The difference is in how x is interpreted. When using a dot, the part after the dot must be a valid variable name, and it directly names the property. When using square brackets, the expression between the brackets is evaluated to get the property name. Whereas value.x fetches the property of value named “x”, value[x] tries to evaluate the expression x and uses the result as the property name.

So if you know that the property you are interested in is called “length”, you say value.length . If you want to extract the property named by the value held in the variable i , you say value[i] . And because property names can be any string, if you want to access a property named “2” or “John Doe”, you must use square brackets: value[2] or value["John Doe"] . This is the case even though you know the precise name of the property in advance, because neither “2” nor “John Doe” is a valid variable name and so cannot be accessed through dot notation.

The elements in an array are stored in properties. Because the names of these properties are numbers and we often need to get their name from a variable, we have to use the bracket syntax to access them. The length property of an array tells us how many elements it contains. This property name is a valid variable name, and we know its name in advance, so to find the length of an array, you typically write array.length because that is easier to write than array["length"] .

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.