类型、值和变量

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Math.pow(2,53) // => 9007199254740992: 2 to the power 53
Math.round(.6) // => 1.0: round to the nearest integer
Math.ceil(.6) // => 1.0: round up to an integer
Math.floor(.6) // => 0.0: round down to an integer
Math.abs(-5) // => 5: absolute value
Math.max(x,y,z) // Return the largest argument
Math.min(x,y,z) // Return the smallest argument
Math.random() // Pseudo-random number x where 0 <= x < 1.0
Math.PI // π: circumference of a circle / diameter
Math.E // e: The base of the natural logarithm
Math.sqrt(3) // The square root of 3
Math.pow(3, 1/3) // The cube root of 3
Math.sin(0) // Trigonometry: also Math.cos, Math.atan, etc.
Math.log(10) // Natural logarithm of 10
Math.log(100)/Math.LN10 // Base 10 logarithm of 100
Math.log(512)/Math.LN2 // Base 2 logarithm of 512
Math.exp(3) // Math.E cubed

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0/0 //=>NaN
Math.sqrt(-1) //=>NaN
1 - 'a' //=>NaN  对不能转换为数字的运算对象使用算术运算符结果为NaN
Infinity + Infinity //=>Infinity
Infinity - Infinity //=>NaN
Infinity * Infinity //=>Infinity
Infinity / Infinity //=>NaN

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Infinity // A read/write variable initialized to Infinity.
Number.POSITIVE_INFINITY // Same value, read-only.
1/0 // This is also the same value.
Number.MAX_VALUE + 1 // This also evaluates to Infinity.
Number.NEGATIVE_INFINITY // These expressions are negative infinity.
-Infinity
-1/0
-Number.MAX_VALUE - 1
NaN // A read/write variable initialized to NaN.
Number.NaN // A read-only property holding the same value.
0/0 // Evaluates to NaN.
Number.MIN_VALUE/2 // Underflow: evaluates to 0
-Number.MIN_VALUE/2 // Negative zero
-1/Infinity // Also negative 0
-0

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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

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var then = new Date(2010, 0, 1); // The 1st day of the 1st month of 2010
var later = new Date(2010, 0, 1, 17, 10, 30); // Same day, at 5:10:30pm, local time
var now = new Date(); // The current date and time
var elapsed = now - then; // Date subtraction: interval in milliseconds
later.getFullYear() // => 2010
later.getMonth() // => 0: zero-based months
later.getDate() // => 1: one-based days
later.getDay() // => 5: day of week. 0 is Sunday 5 is Friday.
later.getHours() // => 17: 5pm, local time
later.getUTCHours() // hours in UTC time; depends on timezone

later.toString() // => "Fri Jan 01 2010 17:10:30 GMT-0800 (PST)"
later.toUTCString() // => "Sat, 02 Jan 2010 01:10:30 GMT"
later.toLocaleDateString() // => "01/01/2010"
later.toLocaleTimeString() // => "05:10:30 PM"
later.toISOString() // => "2010-01-02T01:10:30.000Z"; ES5 only

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\0 The NUL character ( \u0000 )
\b Backspace ( \u0008 )
\t Horizontal tab ( \u0009 )
\n Newline ( \u000A )
\v Vertical tab ( \u000B )
\f Form feed ( \u000C )
\r Carriage return ( \u000D )
\" Double quote ( \u0022 )
\' Apostrophe or single quote ( \u0027 )
\\ Backslash ( \u005C )
\x XX The Latin-1 character specified by the two hexadecimal digits XX
\u XXXX The Unicode character specified by the four hexadecimal digits XXXX

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var s = "hello, world" // Start with some text.
s.charAt(0) // => "h": the first character.
s.charAt(s.length-1) // => "d": the last character.
s.substring(1,4) // => "ell": the 2nd, 3rd and 4th characters.
s.slice(1,4) // => "ell": same thing
s.slice(-3) // => "rld": last 3 characters
s.indexOf("l") // => 2: position of first letter l.
s.lastIndexOf("l") // => 10: position of last letter l.
s.indexOf("l", 3) // => 3: position of first "l" at or after 3
s.split(", ") // => ["hello", "world"] split into substrings
s.replace("h", "H") // => "Hello, world": replaces all instances
s.toUpperCase() // => "HELLO, WORLD"

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s = "hello, world";
s[0] // => "h"
s[s.length-1] // => "d"

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/^HTML/ // Match the letters HTML at the start of a string
/1-9*/ // Match a non-zero digit, followed by any # of digits
/\bjavascript\b/i // Match "javascript" as a word, case-insensitive

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var text = "testing: 1, 2, 3"; // Sample text
var pattern = /\d+/g // Matches all instances of one or more digits
pattern.test(text) // => true: a match exists
text.search(pattern) // => 9: position of first match
text.match(pattern) // => ["1", "2", "3"]: array of all matches
text.replace(pattern, "#"); // => "testing: #, #, #"
text.split(/\D+/); // => ["","1","2","3"]: split on non-digits

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undefined
null
0
-0
NaN
"" // the empty string

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var global = this; // Define a global variable to refer to the global object

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var s = "test"; // Start with a string value.
s.len = 4; // Set a property on it.
var t = s.len; // Now query the property. t is undefined

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function equalArrays(a,b) {
    if (a.length != b.length) return false; // Different-size arrays not equal
    for(var i = 0; i < a.length; i++) // Loop through all elements
        if (a[i] !== b[i]) return false; // If any differ, arrays not equal
    return true; // Otherwise they are equal
}

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10 + " objects" // => "10 objects". Number 10 converts to a string
"7" * "4" // => 28: both strings convert to numbers

var n = 1 - "x"; // => NaN: string "x" can't convert to a number
n + " objects" // => "NaN objects": NaN converts to string "NaN"

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null == undefined // These two values are treated as equal.
"0" == 0 // String converts to a number before comparing.
0 == false // Boolean converts to number before comparing.
"0" == false // Both operands convert to numbers before comparing.

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x + "" // Same as String(x)
+x // Same as Number(x). You may also see x-0
!!x // Same as Boolean(x). Note double !

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var n = 17;
binary_string = n.toString(2); // Evaluates to "10001"
octal_string = "0" + n.toString(8); // Evaluates to "021"
hex_string = "0x" + n.toString(16); // Evaluates to "0x11"

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var n = 123456.789;
n.toFixed(0); // "123457"
n.toFixed(2); // "123456.79"
n.toFixed(5); // "123456.78900"
n.toExponential(1); // "1.2e+5"
n.toExponential(3); // "1.235e+5"
n.toPrecision(4); // "1.235e+5"
n.toPrecision(7); // "123456.8"
n.toPrecision(10); // "123456.7890"

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parseInt("3 blind mice") // => 3
parseFloat(" 3.14 meters") // => 3.14
parseInt("-12.34") // => -12
parseInt("0xFF") // => 255
parseInt("0xff") // => 255
parseInt("-0XFF") // => -255
parseFloat(".1") // => 0.1
parseInt("0.1") // => 0
parseInt(".1") // => NaN: integers can't start with "."
parseFloat("$72.47"); // => NaN: numbers can't start with "$"

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parseInt("11", 2); // => 3 (12 + 1)
parseInt("ff", 16); // => 255 (1516 + 15)
parseInt("zz", 36); // => 1295 (3536 + 35)
parseInt("077", 8); // => 63 (78 + 7)
parseInt("077", 10); // => 77 (7*10 + 7)

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if (new Boolean(false)) console.log(true); //=> true

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[1,2,3].toString() // => "1,2,3"
(function(x) { f(x); }).toString() // => "function(x) {\n f(x);\n}"
/\d+/g.toString() // => "/\d+/g"
new Date(2010,0,1).toString() // => "Fri Jan 01 2010 00:00:00 GMT-0800 (PST)"

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var d = new Date(2010, 0, 1); // January 1st, 2010, (Pacific time)
d.valueOf() // => 1262332800000

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var now = new Date(); // Create a Date object
typeof (now + 1) // => "string": + converts dates to strings
typeof (now - 1) // => "number": - uses object-to-number conversion
now == now.toString() // => true: implicit and explicit string conversions
now > (now -1) // => true: > converts a Date to a number

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var i;
var sum;
var i, sum;
var message = "hello";
var i = 0, j = 0, k = 0;

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var scope = "global"; // Declare a global variable
function checkscope() {
    var scope = "local"; // Declare a local variable with the same name
    return scope; // Return the local value, not the global one
}
checkscope() // => "local"

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scope = "global"; // Declare a global variable, even without var.
function checkscope2() {
    scope = "local"; // Oops! We just changed the global variable.
    myscope = "local"; // This implicitly declares a new global variable.
    return [scope, myscope]; // Return two values.
}
checkscope2() // => ["local", "local"]: has side effects!
scope // => "local": global variable has changed.
myscope // => "local": global namespace cluttered up.

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var scope = "global scope"; // A global variable
function checkscope() {
    var scope = "local scope"; // A local variable
    function nested() {
        var scope = "nested scope"; // A nested scope of local variables
        return scope; // Return the value in scope here
    }
    return nested();
}
checkscope() // => "nested scope"

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var scope = "global";
function f() {
    console.log(scope); // Prints "undefined", not "global"
    var scope = "local"; // Variable initialized here, but defined everywhere
    console.log(scope); // Prints "local"
}

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var truevar = 1; // A properly declared global variable, nondeletable.
fakevar = 2; // Creates a deletable property of the global object.
this.fakevar2 = 3; // This does the same thing.
delete truevar // => false: variable not deleted
delete fakevar // => true: variable deleted
delete this.fakevar2 // => true: variable deleted