Seeing this question: Is there a (built-in) way in JavaScript to check if a string is a valid number? and this: jsperf, one of the presented approaches is this (mutatis mutandis):
var a = "123"
var b = "123b"
if ( +a === +a ) // true
if ( +b === +b ) // false
How does this logic work internally in JavaScript to make this possible?
My question is not how to check if a string is a valid number – this is already answered here: Validate decimal numbers in JavaScript - IsNumeric(). I want to understand how the statement +a === +a works.
+ converts the value to a number.
a gets converted to 123 and 123 === 123.
b gets converted to NaN but NaN !== NaN (because NaN is never equal to another NaN according step 4a of the equality rules).
The + operator here is known as the Unary Plus.
The unary plus operator precedes its operand and evaluates to its operand but attempts to converts it into a number, if it isn't already.
+"123" evaulates as 123.
+a === +a
-> +"123" === +"123"
-> 123 === 123
-> true
+"123b" evaluates as NaN (Not a Number), as the b character cannot be converted with the Unary Plus as without any prefix (like 0x for hexadecimal) the Unary Plus will assume the value is decimal (0-9). NaN is a special case in that it does not compare against anything, including itself:
NaN === NaN
-> false
NaN !== NaN
-> true
Because of this, our +b test case fails:
+b === +b
-> +"123b" === +"123b"
-> NaN === NaN
-> false
If you want both to evaluate to true we can add an isNaN() call at the end:
if ( +a === +a || isNaN(+a) )
if ( +b === +b || isNaN(+b) )
+"123b" is NaN. NaN is not equal to anything, including NaN.