Question

Can someone explain why in javascript,

alert({} == true) shows false

if ({}) alert('true') shows true ?

What’s different in the if condition that changes the result?

I wanted to write some shorthand argument validator obj || (obj = {}); and I was baffled by this discovery.

Answer 1

if ({}) alert('true') -> true

{} is an object, which, when evaluated in the context of an if statement, gets coerced to a Boolean, and since Boolean({}) evaluates to true, you get if (true). This is documented in the ECMAScript specification, Section 12.5 The if Statement:

The production If Statement : if ( Expression ) Statement is evaluated
as follows:

1. Let exprRef be the result of evaluating Expression.
2. If ToBoolean(GetValue(exprRef)) is false, return (normal, empty, empty).
3. Return the result of evaluating Statement.

alert({} == true) -> false

This one is more tricky. From the ECMAScript specification, Section 11.9.3 The Abstract Equality Comparison Algorithm:

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

Thus, {} == true will be evaluated as {} == Number(true), which is evaluated to {} == 1, which is false.

This is also why 1 == true evaluates to true, but 2 == true evaluates to false.