I am having a statement like

var x = parseInt(1)+'b';

why this result is 1b but isNAN('b') is returning true; so result should be NaN?

I am having a statement like

var x = parseInt(1)+'b';

why this result is 1b but isNAN('b') is returning true; so result should be NaN?

• javascript
• nan

• 1 You got the answers already but also check the Documentation on Expressions and operators - String Operators – Nope Commented Jan 10, 2017 at 17:19
• Short answer: because it isn't an arithmetic operation :) – Álvaro González Commented Jan 10, 2017 at 17:29

4 Answers 4

Number + String is not an arithmetic operation.

In the Number + String statement, engine will convert the number into string equivalent.

In your case it will be 1 -> '1'. And then will be simple strings concatenation, because + operator will concatenate two strings.

'1' + 'b' === '1b';

Code Snippet

console.log(1 + 'b');

In case with IsNaN('b'), it tries to detect is the parameter a number or not. 'b' is not a number (NaN), so it will return true.

Code Snippet

console.log(isNaN('b'));

For more see Documentation

Because it's not an arithmetic operation, it's a string concatenation. Whenever either operand to + is a string, string concatenation is done rather than addition; details in the spec. If the other operand isn't a string, it's convered to string. So you end up with "1" + "b" => "1b".

The + operator is defined as concatenation if either operand is a string.

Source: https://www.ecma-international/ecma-262/5.1/#sec-11.6.1

The + operator, when it has a string as one of it's arguments, will be interpreted as a "concatenation" operator instead of "addition". The non-string value will be converted to a string and appended to or prepended to the string value, depending on whether or not it is to the right or the left (respectively) of the operator.