object objects

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object objects

object objects

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object objects, Hidden Object

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 chapters. 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 constants and functions).

Values of the type object are arbitrary collections of properties. One way to create an object is by using braces as an expression.

Inside the braces, there is a list of properties separated by commas. Each property has a name followed by a colon and a value. When an object is written over multiple lines, indenting it like in the example helps with readability. Properties whose names aren’t valid binding names or valid numbers have to be quoted.

This means that braces have two meanings in JavaScript. At the start of a statement, they start a block of statements. In any other position, they describe an object. Fortunately, it is rarely useful to start a statement with an object in braces, so the ambiguity between these two is not much of a problem.

It is possible to assign a value to a property expression with the operator. This will replace the property`s value if it already existed or create a new property on the object if it didn’t.

To briefly return to our tentacle model of bindings—property bindings are similar. They grasp values, but other bindings and properties might be holding onto those same values. You may think of objects as octopuses with any number of tentacles, each of which has a name tattooed on it.

The delete operator cuts off a tentacle from such an octopus. It is a unary operator that, when applied to an object property, will remove the named property from the object. This is not a common thing to do, but it is possible.

The binary in operator, when applied to a string and an object, tells you whether that object has a property with that name. The difference between setting a property to undefined and actually deleting it is that, in the first case, the object still has the property (it just doesn’t have a very interesting value), whereas in the second case the property is no longer present and in will return false.

To find out what properties an object has, you can use the Object.keys function. You give it an object, and it returns an array of strings—the object`s property names.

There`s an Object.assign function that copies all properties from one object into another.

Arrays, then, are just a kind of object specialized for storing sequences of things. If you evaluate typeof [], it produces . You can see them as long, flat octopuses with all their tentacles in a neat row, labeled with numbers.

We will represent the journal that Jacques keeps as an array of objects.

We saw that object values can be modified. The types of values discussed in earlier chapters, such as numbers, strings, and Booleans, are all immutable—it is impossible to change values of those types. You can combine them and derive new values from them, but when you take a specific string value, that value will always remain the same. The text inside it cannot be changed. If you have a string that contains , it is not possible for other code to change a character in your string to make it spell .

Objects work differently. You can change their properties, causing a single object value to have different content at different times.

When we have two numbers, 120 and 120, we can consider them precisely the same number, whether or not they refer to the same physical bits. With objects, there is a difference between having two references to the same object and having two different objects that contain the same properties. Consider the following code:

The object1 and object2 bindings grasp the same object, which is why changing object1 also changes the value of object2. They are said to have the same identity. The binding object3 points to a different object, which initially contains the same properties as object1 but lives a separate life.

Bindings can also be changeable or constant, but this is separate from the way their values behave. Even though number values don’t change, you can use a let binding to keep track of a changing number by changing the value the binding points at. Similarly, though a const binding to an object can itself not be changed and will continue to point at the same object, the contents of that object might change.

When you compare objects with JavaScript`s operator, it compares by identity: it will produce true only if both objects are precisely the same value. Comparing different objects will return false, even if they have identical properties. There is no “deep” comparison operation built into JavaScript, which compares objects by contents, but it is possible to write it yourself (which is one of the exercises at the end of this chapter).

Note that the object added to the journal looks a little odd. Instead of declaring properties like events: events, it just gives a property name. This is shorthand that means the same thing—if a property name in brace notation isn’t followed by a value, its value is taken from the binding with the same name.

We can represent a two-by-two table in JavaScript with a four-element array ([76, 9, 4, 1]). We could also use other representations, such as an array containing two two-element arrays ([[76, 9], [4, 1]]) or an object with property names like and , but the flat array is simple and makes the expressions that access the table pleasantly short. We’ll interpret the indices to the array as two-bit binary numbers, where the leftmost (most significant) digit refers to the squirrel variable and the rightmost (least significant) digit refers to the event variable. For example, the binary number 10 refers to the case where Jacques did turn into a squirrel, but the event (say, “pizza”) didn’t occur. This happened four times. And since binary 10 is 2 in decimal notation, we will store this number at index 2 of the array.

This is a direct translation of the ϕ formula into JavaScript. Math.sqrt is the square root function, as provided by the Math object in a standard JavaScript environment. We have to add two fields from the table to get fields like n1 because the sums of rows or columns are not stored directly in our data structure.

Before finishing the chapter, I want to introduce you to a few more object?related concepts. I’ll start by introducing some generally useful array methods.

Values of type string, number, and Boolean are not objects, and though the language doesn’t complain if you try to set new properties on them, it doesn’t actually store those properties. As mentioned earlier, such values are immutable and cannot be changed.

The Math object is used as a container to group a bunch of related functionality. There is only one Math object, and it is almost never useful as a value. Rather, it provides a namespace so that all these functions and values do not have to be global bindings.

Having too many global bindings “pollutes” the namespace. The more names have been taken, the more likely you are to accidentally overwrite the value of some existing binding. For example, it`s not unlikely to want to name something max in one of your programs. Since JavaScript`s built-in max function is tucked safely inside the Math object, we don’t have to worry about overwriting it.

Back to the Math object. If you need to do trigonometry, Math can help. It contains cos (cosine), sin (sine), and tan (tangent), as well as their inverse functions, acos, asin, and atan, respectively. The number π (pi)—or at least the closest approximation that fits in a JavaScript number—is available as Math.PI. There is an old programming tradition of writing the names of constant values in all caps.

A similar trick works for objects, using braces instead of square brackets.

Because properties only grasp their value, rather than contain it, objects and arrays are stored in the computer`s memory as sequences of bits holding the addresses—the place in memory—of their contents. So an array with another array inside of it consists of (at least) one memory region for the inner array, and another for the outer array, containing (among other things) a binary number that represents the position of the inner array.

JSON looks similar to JavaScript`s way of writing arrays and objects, with a few restrictions. All property names have to be surrounded by double quotes, and only simple data expressions are allowed—no function calls, bindings, or anything that involves actual computation. Comments are not allowed in JSON.

Objects, as generic blobs of values, can be used to build all sorts of data structures. A common data structure is the list (not to be confused with array). A list is a nested set of objects, with the first object holding a reference to the second, the second to the third, and so on.

The resulting objects form a chain, like this:

Building up a list is easier when done back to front. So arrayToList could iterate over the array backwards (see the previous exercise) and, for each element, add an object to the list. You can use a local binding to hold the part of the list that was built so far and use an assignment like list {value: X, rest: list} to add an element.

The operator compares objects by identity. But sometimes you’d prefer to compare the values of their actual properties.

Write a function deepEqual that takes two values and returns true only if they are the same value or are objects with the same properties, where the values of the properties are equal when compared with a recursive call to deepEqual.

The Object.keys function will be useful when you need to go over the properties of objects to compare them.

Your test for whether you are dealing with a real object will look something like typeof x null. Be careful to compare properties only when both arguments are objects. In all other cases you can just immediately return the result of applying.

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