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javascriptcastingtruthiness

Javascript Truthy / Falsy Operation

I have a question regarding javascript truthy / falsy

As far as I know, any non-zero number including negative numbers is truthy. But if this is the case then why

-1 == true //returns false

But also

-1 == false //returns false

Can someone shed some light? I would appreciate it.

Solution

  • When using the == operator with a numeric operand and a boolean operand, the boolean operand is first converted to a number, and the result is compared with the numeric operand. That makes your statements the equivalent of:

    -1 == Number(true)

    and

    -1 == Number(false)

    Which in turn are

    -1 == 1

    and 

    -1 == 0

    Which shows why you're always seeing a false result. If you force the conversion to happen to the numeric operand, you get the result you're after:

    Boolean(-1) == true //true