Part I. Core JavaScript
Ch 2. Lexical Structure
- The
return,break, andcontinuestatements often stand alone, but they are sometimes followed by an identifier or expression. If a line break appears after any of these words (before any other tokens), JavaScript will always interpret that line break as a semicolon.
For example, if you write:
return
true;
JavaScript assumes you meant:
return; true;
Ch 3. Types, Values, and Variables
-
Objects and arrays are mutable. Numbers, booleans, null, and undefined are immutable, strings are immutable too.
-
Arithmetic in JavaScript does not raise errors in cases of overflow, underflow, or divi- sion by zero. Print
Infinityand-Infinityinstead. -
Division by zero is not an error in JavaScript: it simply returns infinity or negative infinity. There is one exception, however: zero divided by zero does not have a well- defined value, and the result of this operation is the special not-a-number value, printed as NaN. NaN also arises if you attempt to divide infinity by infinity, or take the square root of a negative number or use arithmetic operators with non-numeric operands that cannot be converted to numbers.
-
The not-a-number value has one unusual feature in JavaScript: it does not compare equal to any other value, including itself. This means that you can’t write
x == NaNto determine whether the value of a variablexisNaN. Instead, you should writex != x. That expression will be true if, and only if,xisNaN. -
The negative zero value is also somewhat unusual. ```
var zero = 0; // Regular zero
var negz = -0; // Negative zero
zero === negz // => true: zero and negative zero are equal
1/zero === 1/negz // => false: infinity and -infinity are not equal
+ Because of rounding error,the difference between the approximations of `.3` and `.2` is not exactly the same as the difference between the approximations of `.2` and `.1`.
var x = .3 - .2; // thirty cents minus 20 cents
var y = .2 - .1; // twenty cents minus 10 cents
x == y // => false: the two values are not the same!
In JS, `0.3-0.2=0.09999999999999998;`
+ Remember that strings are immutable in JavaScript. Methods like `replace()` and `toUpperCase()` return new strings: they do not modify the string on which they are invoked.
+ Diff between `null` and `undefined`:
typeof(null) // => object
typeof(undefined) // => undefined
null == undefined // =>true
null === undefined // =>false
```
