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I found this code in JavaScript tutorial. It reads if value is null the this.current is assigned to 0, otherwise to a value. How does it work?
I am confused because there is no null or ?? operator in the code.

this.current=value||0;

The Beast
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    Something to note is that null is falsy. – Iman K Dec 03 '15 at 02:45
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    The best explanation can be found here: http://stackoverflow.com/questions/2802055/what-does-the-construct-x-x-y-mean – Marc Young Dec 03 '15 at 02:46
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    Or here http://stackoverflow.com/a/4576915/3791314 because it explains truthy/falsy values – mark Dec 03 '15 at 02:51
  • JavaScript's OR operator doesn't return boolean true/false values like other languages. Rather it checks if the left evaluates to true when type casted to boolean, If true, it returns the value rather than a boolean. If false, then it returns the second value regardless of its value. Thus in JavaScript, the following two statements are equivalent: `$result=$value1||$value2;` `$result=$value1?$value1:$value2;` the weakness with the `?:` syntax is we need to dedicate a statement for assigning the value which isn't possible in the midst of another statement, thus `||` can be very powerful. – Ultimater Dec 03 '15 at 02:57

4 Answers4

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Well, the semantics of the || (logical or) operator is such that as soon as its left side is truthy, it short-circuits and returns that value, otherwise it returns what's on the right side.

That common pattern takes advantage of the semantics by passing a possibly falsy (x) value and a default (y) to the operator: x || y. If x turns out to be non-falsy, that whole expression evaluates to x, otherwise y.

Null is not mentioned there, because null is falsy and the pattern works for all falsy values.

Hunan Rostomyan
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This is how operator || in javascript work. Instead of returning a boolean value, it returns either operand depending on whether they are true or not.

If the first operand is a "true" value, it returns its value directly without even looking at the other operand, else it just returns the other operand's value.

shenkwen
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It is simple, On the right side you have value || 0 That evaluates to the this.current. What you need to understand is the right side evaluates the boolean OR first. That is how JS || supposed to work. It return the value instead of TRUE/FALSE if they are not boolean.

Fawzan
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If you compare something using === it will compare the value and type. For example 

var a = false;
If(a == null){
   //triggered
}

If(a === null){
 //not works
 }
Jake Psimos
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