JavaScript Type Conversion
Converting Strings to Numbers
Converting Numbers to Strings
Converting Dates to Numbers
Converting Numbers to Dates
Converting Booleans to Numbers
Converting Numbers to Booleans
JavaScript Type Conversion
JavaScript variables can be converted to a new variable and another data type:
By the use of a JavaScript function
Automatically by JavaScript itself
Converting Strings to Numbers
The global method Number() converts a variable (or a value) into a number.
A numeric string (like "3.14") converts to a number (like 3.14).
An empty string (like "") converts to 0.
A non numeric string (like "John") converts to NaN (Not a Number).
Examples
These will convert:
Number("3.14")
Number(Math.PI)
Number(" ")
Number("")
These will not convert:
Number("99 88")
Number("John")
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Number Methods
In the chapter Number Methods, you
will find more methods that can be used to convert strings to
numbers:
Method
Description
Number()
Returns a number, converted from its argument
parseFloat()
Parses a string and returns a floating point number
parseInt()
Parses a string and returns an integer
The Unary + Operator
The unary + operator can be used to
convert a variable to a number:
Example
let y = "5";
// y is a string
let x = + y;
// x is a number
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If the
variable cannot be converted, it will still become a number, but with the value NaN
(Not a Number):
Example
let y = "John";
// y is a string
let x = + y; // x is a number (NaN)
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Converting Numbers to Strings
The global method String() can convert numbers to strings.
It can be used on any type of numbers, literals, variables, or expressions:
Example
String(x) // returns a string from a number variable x
String(123) // returns a string from a number literal 123
String(100 + 23) // returns a string from a number from an expression
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The Number method toString() does the same.
Example
x.toString()(123).toString()(100 + 23).toString()
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More Methods
In the chapter Number Methods, you
will find more methods that can be used to convert numbers to
strings:
Method
Description
toExponential()
Returns a string, with a number rounded and written using exponential notation.
toFixed()
Returns a string, with a number rounded and written with a specified number of decimals.
toPrecision()
Returns a string, with a number written with a specified length
Converting Dates to Numbers
The global method Number() can be used to convert dates to
numbers.
d = new Date();Number(d) // returns 1404568027739
The date method getTime() does the same.
d = new Date();d.getTime() // returns 1404568027739
Converting Dates to Strings
The global method String() can convert dates to
strings.
String(Date()) // returns "Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)"
The Date method toString() does the same.
Example
Date().toString() // returns "Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)"
In the chapter Date Methods, you
will find more methods that can be used to convert dates to
strings:
Method
Description
getDate()
Get the day as a number (1-31)
getDay()
Get the weekday a number (0-6)
getFullYear()
Get the four digit year (yyyy)
getHours()
Get the hour (0-23)
getMilliseconds()
Get the milliseconds (0-999)
getMinutes()
Get the minutes (0-59)
getMonth()
Get the month (0-11)
getSeconds()
Get the seconds (0-59)
getTime()
Get the time (milliseconds since January 1, 1970)
Converting Booleans to Numbers
The global method Number() can also convert booleans to numbers.
Number(false) // returns 0
Number(true) // returns 1
Converting Booleans to Strings
The global method String() can convert booleans to
strings.
String(false) // returns "false"
String(true) // returns "true"
The Boolean method toString() does the same.
false.toString() // returns "false"
true.toString() // returns "true"
Automatic Type Conversion
When JavaScript tries to operate on a "wrong" data type, it will try to
convert the value to a "right" type.
The result is not always what you expect:
5 + null // returns 5 because null is converted to 0"5" + null // returns "5null" because null is converted to "null""5" + 2 // returns
"52" because 2 is converted to "2"
"5" - 2 // returns 3 because "5" is converted to 5"5" * "2" // returns
10 because "5" and "2" are
converted to 5 and 2
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Automatic String Conversion
JavaScript automatically calls the variable's toString() function when you try
to "output" an object or a variable:
document.getElementById("demo").innerHTML = myVar;
// if myVar = {name:"Fjohn"} // toString converts to "[object Object]"
// if myVar = [1,2,3,4] // toString converts to "1,2,3,4"
// if myVar = new Date() // toString converts to "Fri Jul 18 2014 09:08:55 GMT+0200"
Numbers and booleans are also converted, but this is not very visible:
// if myVar = 123 // toString converts to "123"
// if myVar = true // toString converts to "true"
// if myVar = false // toString converts to "false"
JavaScript Type Conversion Table
This table shows the result of converting different JavaScript values to Number, String, and Boolean:
OriginalValue
Convertedto Number
Convertedto String
Convertedto Boolean
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false
0
"false"
false
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true
1
"true"
true
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0
0
"0"
false
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1
1
"1"
true
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"0"
0
"0"
true
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"000"
0
"000"
true
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"1"
1
"1"
true
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NaN
NaN
"NaN"
false
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Infinity
Infinity
"Infinity"
true
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-Infinity
-Infinity
"-Infinity"
true
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""
0
""
false
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"20"
20
"20"
true
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"twenty"
NaN
"twenty"
true
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[ ]
0
""
true
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[20]
20
"20"
true
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[10,20]
NaN
"10,20"
true
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["twenty"]
NaN
"twenty"
true
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["ten","twenty"]
NaN
"ten,twenty"
true
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function(){}
NaN
"function(){}"
true
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{ }
NaN
"[object Object]"
true
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null
0
"null"
false
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undefined
NaN
"undefined"
false
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Values in quotes indicate string values.
Red values indicate values (some) programmers might not expect.
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Reference: https://www.w3schools.com/js/js_type_conversion.asp