javascript中的短路评估

Question

I am in a need to implement short circuit evaluation logic in my code. And I read some articles http://en.wikipedia.org/wiki/Short-circuit_evaluation , http://www.grauw.nl/blog/entry/510 to understand about it . But they haven't covered in detail and I tried up some examples.

Ex:

function a() {return true;}

function b() {return false;}

function c() {return true;}

Expression

Case 1) a() && b() && c();

Case 2) a() || b() && c(); a() || b() && c();

Case 3) a() && c() || b(); a() && c() || b();

Case 1:

c() will not be executed.

Case 2:

b() and c() will not be executed

Case 3:

b() will not be executed.

Where should we learn about this short-circuit evaluation?

Answer 1

Where should we learn about this short-circuit evaluation?

The specification is handy, as is MDN .

But the short version is:

• The left-hand operand to && or || is always evaluated, giving us the left-hand value .

• With && , if the left-hand value is falsey, the right-hand operand is not evaluated; the expression's value is the left-hand value.

• With || , if the left-hand value is truthy, the right-hand operand is not evaluated; the expression's value is the left-hand value.

• Otherwise, the right-hand operand is evaluated and the expression's result is its value.

"Falsey" values are any value that coerces to false when used as a boolean. Those are 0 , "" , NaN , undefined , null , and of course, false .

"Truthy" values are any values that aren't falsey.

More on my blog: JavaScript's Curiously-Powerful OR Operator ( || ) . (I keep meaning to do the && version of that...)