JAVASCRIPT
THE BASICS
Types
Values
Variables
Type
conversion
JavaScript
is very flexible about the types of
values it requires
when
JavaScript
expects a certain value
no matter the value supplied
it will try to convert it as needed
type conversions summary
Value | Converted to | |||
String | Number | Boolean | Object | |
undefined | "undefined" | NaN | false | throws TypeError |
null | "null" | 0 | false | throws TypeError |
true | "true" | 1 | new Boolean(true) | |
false | "false" | 0 | new Boolean(false) | |
"" (empty string) | 0 | false | new String("") | |
"1.2" (nonempty, numeric) | 1.2 | true | new String("1.2") | |
"one" (nonempty, non-numeric) | NaN | true | new String("one") | |
0 | "0" | false | new Number(0) | |
-0 | "0" | false | new Number(-0) | |
NaN | "NaN" | false | new Number(NaN) | |
Infinity | "Infinity" | true | new Number(Infinity) | |
-Infinity | "-Infinity" | true | new Number(-Infinity) | |
1 (finite, non-zero) | "1" | true | new Number(1) | |
{} (any object) | read more | read more | true | |
[] (empty array) | "" | 0 | true | |
[1] (one numeric element) | "9" | 9 | true | |
['str'] (any other array) | use join() method | NaN | true | |
function(){} (any function) | read more | NaN | true |
Primitive to primitive
conversions
1 + ' string' //=> "1 string" -> Number becomes a string
"1" / "2" //=> 0.5 -> both strings become numbers
1 / "two" //=> NaN -> string can't be converted to number
NaN + ' string' //=> "NaN string" -> NaN becomes a string
true + 1 + null //=> 2 -> true becomes 1, null becomes 0 (1 + 1 + 0 = 2)
1 / false //=> Infinity -> false becomes 0 (1 / 0 = Infinity)
['Java'] + 'Script' //=> "JavaScript" -> array values become a string
PRIMITIVE TO OBJECT
CONVERSIONs
primitives convert to their wrapper object
as if by calling the
String() / Number() / Boolean()
constructor functions
BUT
null & undefined are an exception
they raise a TypeError if used where an object is expected
OBJECT TO PRIMITIVE
CONVERSIONs
even
wrapper objects
var obj = new Boolean(false);
if (obj) {
console.log(true); //=> true -> new Boolean(false) is an object & converts to true
} else {
console.log(false);
}
BUT
object-to-string & object-to-number conversions
are more complicated
objects
inherit two different methods
that perform these conversions
toString()
returns a string representation of the object
valueOf()
converts an object to a primitive value
that represents the object
toString() method
var obj = {
title: 'JavaScript - The Basics'
};
obj.toString(); //=> "[object Object]"
other classes define more specific versions of this method
[1, 2, 3, 4].toString(); //=> "1,2,3,4"
var fn = function (param) {
return param;
};
fn.toString(); //=> "function (param) { return param; }"
var time = new Date();
time.toString(); //=> "Tue Aug 12 2014 21:07:31 GMT+0300 (GTB Daylight Time)"
valueOf() method
a less well-defined conversion function
because objects are compound values
cannot really be represented by a single primitive value
by default the valueOf() method simply returns the object itself
var obj = {
title: 'JavaScript - The Basics'
};
obj.valueOf(); //=> Object {title: "JavaScript - The Basics"}
[1, 2, 3, 4].valueOf(); //=> [1, 2, 3, 4]
(new Date()).valueOf(); //=> 1407867809210 -> milliseconds since January 1, 1970
Conversions & equality
because JavaScript can convert values flexibly
its == equality operator is also flexible with its notion of equality
all of the following comparisons are true
null == undefined //=> true
"0" == 0 //=> true -> string becomes a number (0 == 0)
0 == false //=> true -> boolean becomes a number (0 == 0)
"0" == false //=> true -> both 0 & false become numbers (0 == 0)
undefined == false //=> false -> == operator never attempts to
// convert its operands to booleans
Explicit conversions
although many type conversions are performed automatically
sometimes explicit conversions are needed
the simplest way to do it is by invoking the functions
Number() / String() / Boolean() and Object()
without the new operator
Number("1"); // => 1
String(true); // => "true"
Boolean({}); // => true
Object(1); // => Number {} -> new Number(1)
Object(); // => Object {} -> an empty object
some operators perform implicit type conversions
&
are sometimes used for this purposes
1 + "" //=> "1" -> + converts the second operator to string if one operator is a string
+"1" //=> 1 -> unary + operator converts its operand to number
!!"" //=> false -> unary ! operator converts its operand to a boolean and negates it
JavaScript
toString()
defined by the Number class
converts a number to a string
accepts optional argument as radix for the conversion
var n = 15;
n.toString(2); //=> "1111"
n.toString(8); //=> "17"
n.toString(16); //=> "f"
toFixed()
converts a number to a string
with a specified number of digits after the decimal point
var n = 12345.6789;
n.toFixed(0); //=> "12345"
n.toFixed(2); //=> "12345.68"
n.toFixed(7); //=> "12345.6789000"
toExponential()
converts a number to a string using exponential notation
with one digit before the decimal point & a specified number of digits
after the decimal point
var n = 12345.6789;
n.toExponential(0); //=> "1e+4"
n.toExponential(2); //=> "1.23e+4"
n.toExponential(7); //=> "1.2345679e+4"
toPrecision()
converts a number to a string
with the number of significant digits you specify
var n = 12345.6789;
n.toPrecision(1); //=> "1e+4"
n.toPrecision(7); //=> "12345.68"
n.toPrecision(21); //=> "12345.6789000000007945"
parseFloat()
parses integers and floating-point numbers
parseFloat("1px"); //=> 1
parseFloat(".1"); //=> 0.1
parseFloat("$1.0"); //=> NaN
parseInt()
parses only integers
if string begins with "0x" / "0X" interprets number as hexadecimal
parseInt("1px"); // => 1
parseInt("1.0"); // => 1
parseInt("0.1"); // => 0
parseInt(".1"); // => NaN -> integers can't start with "."
parseInt("0xf"); // => 15
parseInt("0XF"); // => 15
parseInt("1111", 2); // => 15
parseInt("f", 16); // => 15
parseInt("017", 8); // => 15
both parseInt() & parseFloat() functions
skip leading whitespace
JavaScript - The Basics - Types, Values & Variables III
By Raul Matei
JavaScript - The Basics - Types, Values & Variables III
- 1,967