Notes from JavaScript:The Definitive Guide / JavaScript权威指南笔记

Part I. Core JavaScript

Ch 2. Lexical Structure

• The return, break, and continue statements 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 Infinity and -Infinity instead.

• 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 == NaN to determine whether the value of a variable x is NaN. Instead, you should write x != x. That expression will be true if, and only if, x is NaN.

• 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
  

Ch 4. Expressions and Operators

To be Continued …

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