Why does "true" == true show false in JavaScript?
MDC describes the ==
operator as follows:
If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible.
With this in mind, I would evaluate "true" == true
as follows:
- Are they of the same type? No
- Is either operand a number or boolean? Yes
- Can we convert both to a number? No (
isNaN(Number("true")) // true
) - Is either operand a string? Yes
- Can we convert the other operand to a string? Yes (
String(true) === "true" // true
)
I've ended up with the strings "true"
and "true"
, which should evaluate to true
, but JavaScript shows false.
What have I missed?
Solution 1:
Because "true"
is converted to NaN
, while true
is converted to 1
. So they differ.
Like you reported, both are converted to numbers, because at least true
can be (see Erik Reppen's comment), and then compared.
Solution 2:
The ==
comparison operator is defined in ECMA 5 as:
- If Type(x) is Number and Type(y) is String,
return the result of the comparison x == ToNumber(y). - If Type(x) is String and Type(y) is Number,
return the result of the comparison ToNumber(x) == y. - If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
- If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
So, "true" == true is evaluated as:
- "true" == ToNumber(true) (via rule 7)
- "true" == 1
- ToNumber("true") == 1 (via rule 5)
- NaN == 1
===> false
Solution 3:
Acording to The Abstract Equality Comparison Algorithm
http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3
if one of the oprends is a boolean and other is not, boolean is converter to number 0 or 1. so true == "true"
is false.