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I saw this construction in order to get the browser viewport width:
function () { return window.innerWidth || document.documentElement.clientWidth || document.body.clientWidth; } I understand the browser quirks involved. What I don't understand is why || returns the value. So I tried this alert(undefined || 0 || 3); and sure enough, it alerts 3. I find this bizarre, because I expect true or false. Could anyone explain what's going on?
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Treating integers as boolean values C++ does not really have a boolean type; bool is the same as int. Whenever an integer value is tested to see whether it is true of false, 0 is considered to be false and all other integers are considered be true.
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.
Because they don't represent equally convertible types/values. The conversion used by == is much more complex than a simple toBoolean conversion used by if ('true') . So given this code true == 'true' , it finds this: "If Type(x) is Boolean , return the result of the comparison ToNumber(x) == y ."
It started with: Boolean type take only two literal values: true and false. These are distinct from numeric values, so true is not equal to 1, and false is not equal to 0. I know, that every type in JavaScript has a Boolean equivalent.
The JavaScript operator || is defined to return the left value if it evaluates to a truthy value, otherwise the right value instead of returning true itself. That's just how it's defined in the spec.
I know it can be annoying at times, you might accidentally end up holding a reference to something you don't want to hold on to, but it also allows for the handy trick your example has. Everything has its pros and cons.
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Take a look at the ECMAScript standards section 11.11 Binary Logical Operators
The production LogicalORExpression : LogicalORExpression || LogicalANDExpression is evaluated as follows:
1.Evaluate LogicalORExpression.
2.Call GetValue(Result(1)).
3.Call ToBoolean(Result(2)).
4.If Result(3) is true, return Result(2).
5.Evaluate LogicalANDExpression.
6.Call GetValue(Result(5)).
7.Return Result(6).
So it evaluates the boolean conversion of each operand, but returns the actual value of the operand.
If you want to know how Javascript converts values to a boolean, see section 9.2 ToBoolean
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