Values, Types, and Operators :: Eloquent JavaScript - 0 views
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Not all operators are symbols. Some are written as words. One example is the typeof operator, which produces a string value naming the type of the value you give it.
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Having such numbers is useful for storing strings inside a computer because it makes it possible to represent them as a sequence of numbers. When comparing strings, JavaScript goes over them from left to right, comparing the numeric codes of the characters one by one.
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There is only one value in JavaScript that is not equal to itself, and that is NaN, which stands for “not a number”.
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In practice, you can usually get by with knowing that of the operators we have seen so far, || has the lowest precedence, then comes &&, then the comparison operators (>, ==, and so on), and then the rest. This order has been chosen such that, in typical expressions like the following one, as few parentheses as possible are necessary:
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The difference in meaning between undefined and null is an accident of JavaScript’s design, and it doesn’t matter most of the time. In the cases where you actually have to concern yourself with these values, I recommend treating them as interchangeable (more on that in a moment).
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. Yet in the third expression, + tries string concatenation before numeric addition
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When something that doesn’t map to a number in an obvious way (such as "five" or undefined) is converted to a number, the value NaN is produced.
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Further arithmetic operations on NaN keep producing NaN, so if you find yourself getting one of those in an unexpected place, look for accidental type conversions.
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g ==, the outcome is easy to predict: you should get true when both values are the same, except in the case of NaN.
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But when the types differ, JavaScript uses a complicated and confusing set of rules to determine what to do. In most cases, it just tries to convert one of the values to the other value’s type. However, when null or undefined occurs on either side of the operator, it produces true only if both sides are one of null or undefined.
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That last piece of behavior is often useful. When you want to test whether a value has a real value instead of null or undefined, you can simply compare it to null with the == (or !=) operator.
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The rules for converting strings and numbers to Boolean values state that 0, NaN, and the empty string ("") count as false, while all the other values count as true.
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where you do not want any automatic type conversions to happen, there are two extra operators: === and !==. The first tests whether a value is precisely equal to the other, and the second tests whether it is not precisely equal. So "" === false is false as expected.
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The logical operators && and || handle values of different types in a peculiar way. They will convert the value on their left side to Boolean type in order to decide what to do, but depending on the operator and the result of that conversion, they return either the original left-hand value or the right-hand value.
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The || operator, for example, will return the value to its left when that can be converted to true and will return the value on its right otherwise. This conversion works as you’d expect for Boolean values and should do something analogous for values of other types.
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This functionality allows the || operator to be used as a way to fall back on a default value. If you give it an expression that might produce an empty value on the left, the value on the right will be used as a replacement in that case.
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The && operator works similarly, but the other way around. When the value to its left is something that converts to false, it returns that value, and otherwise it returns the value on its right.
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Another important property of these two operators is that the expression to their right is evaluated only when necessary. In the case of true || X, no matter what X is—even if it’s an expression that does something terrible—the result will be true, and X is never evaluated. The same goes for false && X, which is false and will ignore X. This is called short-circuit evaluation.
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- to negate a number
