The JavaScript Global Object
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The details of the Global object in JavaScript
JavaScript provides a global object which has a set of properties, functions and objects that are accessed globally, without a namespace.
The properties are:
Infinity
NaN
undefined
The functions are:
decodeURI()
decodeURIComponent()
encodeURI()
encodeURIComponent()
eval()
isFinite()
isNaN()
parseFloat()
parseInt()
These are the objects:
and errors:
Error
EvalError
RangeError
ReferenceError
SyntaxError
TypeError
URIError
I describe errors on this JavaScript Errors reference post.
Let’s now describe here the global properties and functions.
Infinity
Infinity
in JavaScript is a value that represents infinity.
Positive infinity. To get negative infinity, use the –
operator: -Infinity
.
Those are equivalent to Number.POSITIVE_INFINITY
and Number.NEGATIVE_INFINITY
.
Adding any number to Infinity
, or multiplying Infinity
for any number, still gives Infinity
.
NaN
The global NaN
value is an acronym for Not a Number
. It’s returned by operations such as zero divided by zero, invalid parseInt() operations, or other operations.
parseInt() //NaN
parseInt('a') //NaN
0/0 //NaN
A special thing to consider is that a NaN
value is never equal to another NaN
value. You must use the isNaN()
global function to check if a value evaluates to NaN
:
NaN === NaN //false
0/0 === NaN //false
isNaN(0/0) //true
undefined
The global undefined
property holds the primitive value undefined
.
Running a function that does not specify a return value returns undefined
:
const test = () => {}
test() //undefined
Unlike NaN
, we can compare an undefined
value with undefined
, and get true:
undefined === undefined
It’s common to use the typeof
operator to determine if a variable is undefined:
if (typeof dog === 'undefined') {
}
decodeURI()
Performs the opposite operation of encodeURI()
decodeURIComponent()
Performs the opposite operation of encodeURIComponent()
encodeURI()
This function is used to encode a complete URL. It does encode all characters to their HTML entities except the ones that have a special meaning in a URI structure, including all characters and digits, plus those special characters:
~!@#$&*()=:/,;?+-_.
Example:
encodeURI("http://flaviocopes.com/ hey!/")
//"http://flaviocopes.com/%20hey!/"
encodeURIComponent()
Similar to encodeURI()
, encodeURIComponent()
is meant to have a different job.
Instead of being used to encode an entire URI, it encodes a portion of a URI.
It does encode all characters to their HTML entities except the ones that have a special meaning in a URI structure, including all characters and digits, plus those special characters:
-_.!~*'()
Example:
encodeURIComponent("http://www.example.org/a file with spaces.html")
// "http%3A%2F%2Fwww.example.org%2Fa%20file%20with%20spaces.html"
eval()
This is a special function that takes a string that contains JavaScript code, and evaluates / runs it.
This function is very rarely used and for a reason: it can be dangerous.
I recommend to read this article on the subject.
isFinite()
Returns true if the value passed as parameter is finite.
isFinite(1) //true
isFinite(Number.POSITIVE_INFINITY) //false
isFinite(Infinity) //false
isNaN()
Returns true if the value passed as parameter evaluates to NaN
.
isNaN(NaN) //true
isNaN(Number.NaN) //true
isNaN('x') //true
isNaN(2) //false
isNaN(undefined) //true
This function is very useful because a NaN
value is never equal to another NaN
value. You must use the isNaN()
global function to check if a value evaluates to NaN
:
0/0 === NaN //false
isNaN(0/0) //true
parseFloat()
Like parseInt()
, parseFloat()
is used to convert a string value into a number, but retains the decimal part:
parseFloat('10,000', 10) //10 ❌
parseFloat('10.00', 10) //10 ✅ (considered decimals, cut)
parseFloat('10.000', 10) //10 ✅ (considered decimals, cut)
parseFloat('10.20', 10) //10.2 ✅ (considered decimals)
parseFloat('10.81', 10) //10.81 ✅ (considered decimals)
parseFloat('10000', 10) //10000 ✅
parseInt()
This function is used to convert a string value into a number.
Another good solution for integers is to call the parseInt()
function:
const count = parseInt('1234', 10) //1234
Don’t forget the second parameter, which is the radix, always 10 for decimal numbers, or the conversion might try to guess the radix and give unexpected results.
parseInt()
tries to get a number from a string that does not only contain a number:
parseInt('10 lions', 10) //10
but if the string does not start with a number, you’ll get NaN
(Not a Number):
parseInt("I'm 10", 10) //NaN
Also, just like Number it’s not reliable with separators between the digits:
parseInt('10,000', 10) //10 ❌
parseInt('10.00', 10) //10 ✅ (considered decimals, cut)
parseInt('10.000', 10) //10 ✅ (considered decimals, cut)
parseInt('10.20', 10) //10 ✅ (considered decimals, cut)
parseInt('10.81', 10) //10 ✅ (considered decimals, cut)
parseInt('10000', 10) //10000 ✅
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