Strange behaviour of javascript while chaining math comparison operators

Question

When checking in chrome console running following statements return strange results.

0.5 < 0.6 < 0.7 => returns false

1.5 < 1.6 < 1.7 => return true

Can anyone explain this behaviour?

alert("0.5 < 0.6 < 0.7  = " + (0.5 < 0.6 < 0.7));
alert("1.5 < 1.6 < 1.7  = " + (1.5 < 1.6 < 1.7));

I don't like these questions. There's already a million of them, someone meddling in a console and then being surprised. There's an infinity of possible combinations that you can run into console and ask SO question about it. — Tomáš Zato, Jan 08 '16 at 10:47

thefourtheye · Accepted Answer · 2016-01-08T10:46:24.380

The expression

0.5 < 0.6 < 0.7

is evaluated, like this

(0.5 < 0.6) < 0.7

Since the first part is true, it will be effectively

true < 0.7

Both the sides will be tried to be converted to a number and since true is 1 when converted to a number, the expression becomes

1 < 0.7

which is not correct. That is why false is returned.

Same way, in the second case,

1.5 < 1.6

is true, so it becomes 1 and the expression becomes

1 < 1.7

which is true.

You can check the behaviour of booleans as numbers, like this

console.log(+true);
// 1
console.log(+false);
// 0
console.log(Number(true));
// 1
console.log(Number(false));
// 0

score 1 · Answer 2 · answered Jan 08 '16 at 10:36

1

0.5 < 0.6 // true

true became 1, because of the type changing to number

1 < 0.7 // false

the second one:

1.5 < 1.6 // true

true became 1, because of the type changing

1 < 1.7 // true

answered Jan 08 '16 at 10:36

Nina Scholz

score 0 · Answer 3 · answered Jan 08 '16 at 10:39

The associativity of < is from left to right. And a boolean true has a numerical value of 1, and false has a value of 0.

Putting this together:

where the parentheses make the associativity clear.

3 Answers3