Simply put: Type Coercion is the process of converting from one data type to another(be it a number, string or a boolean). Coercion can be necessary for cases where you have a number in the form of a string(say, "67"
), and you need to perform some numeric operations on it, for example.
In this article, we're going to look at how to convert from a data type to a number or string in JavaScript. You cannot(you can, but it's a bit clunky ðŸ˜) convert something to an array or object since they are not primitives.
String/Boolean → Number
To convert a string or Boolean to a number, there are 3 possible ways:
parseInt()
and parseFloat()
function
The parseInt()
and parseFloat()
methods are built-in to the global or window object in JavaScript. These methods take in a string and try to parse the value from the first character until the string has a valid numeral.
The parseInt
function returns only the integer part, while parseFloat
function returns whatever is parsed, with the decimal part.
You can also specify an optional parameter radix
(only available in parseInt), which tells the function the numeral system to use while parsing. If nothing is specified, the following conditions are applicable:
- If the string starts with "0x", then the radix parameter will default to 16(hexadecimal)
- If the string starts with 0, then the radix parameter will default to 8(octal). This behaviour is deprecated since ES6, where it will default to 10 unless specified otherwise.
- If the above conditions are not met, the value of the radix parameter will be 10.
If the value cannot be parsed, the function will return NaN
(not a number).
Usage:
parseInt("123582jdme"); // 123582
parseInt("37.19"); // 37
parseFloat("37.19"); // 37.19
parseInt("0x8F"); // 143
parseInt("abcr2929"); // NaN
parseInt(true); // NaN
parseInt(10n); // 10
Number
constructor
The Number
constructor function or "primitive object wrapper" is, again, a built-in one in JavaScript. It takes in any primitive type and converts it into a number. It can take a boolean or string(or even a number!) and tries to convert it into a numeric value. You need not use the new
keyword with it, since it is a primitive type wrapper. Like parseInt
and parseFloat
, you can pass into it a hexadecimal number starting with "0x", and if it cannot be converted, it returns NaN
Usage:
Number("0x89FAC"); // 565164
Number("0x0F"); // 15
Number(true); // 1
Number(false); // 0
Number(null); // 0
Number("123abc"); // NaN
Number(10n); // 10
If you notice the last line, it returns NaN if 123abc is tried to be converted. If you are using parseInt
, this will be 123. This is because parseInt
and parseFloat
coerce a string to a number "from the first non-whitespace character until a valid numeral", while the Number
function tries to convert the number in a whole.
Also, you may notice that when a boolean is passed to parseInt
or parseFloat
, it returns NaN
, while Number
returns a number, 1 or 0, depending on the boolean. This has to do with "auto-coercion". Any thing passed to parseInt
or parseFloat
will be coerced to a string behind the scenes, so true
will be coerced to "true"
and false
will be coerced to "false"
.
The unary plus(+) operator
The unary plus operator is used before its operand. The expression will be evaluated to the numeric form of the operand(if not already a number). Again, like Number
, you can have hexadecimal numbers, prefixed with "0x", and the expression will be evaluated to NaN
if the operand cannot be parsed.
Usage:
+"7" // 7
+"-3" // -3
+true // 1
+false // 0
+null // 0
+"abc123" // NaN
+10n // Uncaught TypeError: Cannot convert a BigInt value to a number
Here, you may notice that when we try to convert a BigInt
value to a number using the unary plus, it throws an exception, while the other two methods stated above convert it to regular numbers.
If you don't know what a
BigInt
is, check out the MDN docs.
My fav: Unary plus(+)
Browser support
You can check that out from the caniuse reports:
-
parseInt
andparseFloat
: https://caniuse.com/mdn-javascript_builtins_parseint and https://caniuse.com/mdn-javascript_builtins_parsefloat -
Number
: https://caniuse.com/mdn-javascript_builtins_number - Unary plus: https://caniuse.com/mdn-javascript_operators_unary_plus
Number/Boolean/Array → String
To convert a boolean, number or an array to a string, there are two possible ways:
String
constructor
Like Number
, String
is a primitive object wrapper built-in to JavaScript. It coerces whatever is passed into it, to a string. This is a super straightforward one and doesn't have any sort of weird effects.
Usage:
String(true); // "true"
String(false); // "false"
String(0x0F); // "15"
String(27); // "27"
String(undefined); // "undefined"
String(null); // "null"
String(12948975894798759879867587646); // "1.294897589479876e+28"
String(12948975894798759879867587646n); // "12948975894798759879867587646"
String([1,2,3,true]); // "1,2,3,true"
String({a: 1, b: 2, c: 3}); // '[object Object]'
.toString()
function
The toString()
method is a property in all data types: number, string(!), boolean, array and object and even things like Date, Error, etc. This method, when used, yields the same result as the String
primitive object wrapper, except that the undefined
and null
do not contain any .toString
method(will error out), as they are... undefined
and null
😉
Usage:
true.toString(); // "true"
false.toString(); // "false"
(0x0F).toString(); // "15"
(27).toString(); // "27"
(12948975894798759879867587646).toString(); // "1.294897589479876e+28"
(12948975894798759879867587646n).toString(); // "12948975894798759879867587646"
[1,2,3,true].toString(); // "1,2,3,true"
({a: 1, b: 2, c: 3}).toString(); // '[object Object]'
Conclusion
Aaaaaannnd, that's it for this one. I hope you've enjoyed this article and learnt something new.
Thank you and have a great day!
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