If isNaN(x) returns true, x will get coerced to NaN and make most arithmetic expressions return NaN (because NaN propagates). If isNaN(x) returns false, you can use x in an arithmetic expression as if it's a valid number that's not NaN. The isNaN() function answers the question "is the input functionally equivalent to NaN when used in a number context". To test if a value is a number, use typeof x = "number". Alternatively, the expression x != x can be used, and neither of the solutions is subject to the false positives that make the global isNaN() unreliable. You can check if the value is NaN by using the method IsNaN or Number. The NaN can appear when you convert an Invalid value to a number, Divide Zero by Zero, Some operations involving infinity & all Operations involving NaN, etc. Number.isNaN() is a more reliable way to test whether a value is the number value NaN or not. For Example, converting a string to a number. Therefore, isNaN() answers neither the question "is the input the floating point NaN value" nor the question "is the input not a number". This behavior of isNaN() for non-numeric arguments can be confusing! For example, an empty string is coerced to 0, while a boolean is coerced to 0 or 1 both values are intuitively "not numbers", but they don't evaluate to NaN, so isNaN() returns false. When the argument to the isNaN() function is not of type Number, the value is first coerced to a number, and the resulting value is then compared against NaN. IsNaN() is a function property of the global object.įor number values, isNaN() tests if the number is the value NaN. Warning: unreachable code after return statement.Warning: -file- is being assigned a //# sourceMappingURL, but already has one.TypeError: X.prototype.y called on incompatible type.TypeError: setting getter-only property "x".TypeError: Reduce of empty array with no initial value.TypeError: property "x" is non-configurable and can't be deleted.TypeError: invalid assignment to const "x".TypeError: invalid 'instanceof' operand 'x'.TypeError: cannot use 'in' operator to search for 'x' in 'y'.TypeError: can't redefine non-configurable property "x".TypeError: can't delete non-configurable array element.TypeError: can't define property "x": "obj" is not extensible.TypeError: can't convert BigInt to number.TypeError: can't assign to property "x" on "y": not an object.TypeError: "x" is not a non-null object.Synta圎rror: Using to indicate sourceURL pragmas is deprecated.Synta圎rror: unterminated string literal.Synta圎rror: unparenthesized unary expression can't appear on the left-hand side of '**'.Synta圎rror: unlabeled break must be inside loop or switch.Synta圎rror: Unexpected '#' used outside of class body.Synta圎rror: test for equality (=) mistyped as assignment (=)?.Synta圎rror: redeclaration of formal parameter "x".Synta圎rror: missing = in const declaration.Unicode character class escape: \p after property list.Character class escape: \d, \D, \w, \W, \s, \S.Enumerability and ownership of properties.
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