In the following javascript function what does the || operator do? [duplicate]

This qu开发者_StackOverflow社区estion already has answers here: JavaScript OR (||) variable assignment explanation (12 answers) Closed 7 years ago.

Taken from a box2djs sample.

I'm trying to understand the library, but I do not understand the line:

    ballSd.radius = rad || 10;

what does it mean?

Here's the full definition

createBall2 = function(world, x, y, rad, fixed) {
    var ballSd = new b2CircleDef();
    if (!fixed) ballSd.density = 1.0;

    // what does the next line do?
    ballSd.radius = rad || 10;

    ballSd.restitution = 0.2;
    var ballBd = new b2BodyDef();
    ballBd.AddShape(ballSd);
    ballBd.position.Set(x,y);
    return world.CreateBody(ballBd);
};

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ballSd.radius = rad || 10; 

means: if rad == true (or truthy) return the value of rad, otherwise return 10

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A boolean expression in JavaScript does not return false or true^†, but the first operand (from left to right) that determines the outcome of the expression.

When using logical OR ||, this is the first operand that evaluates to true (similarly the first operand that evaluates to false for &&).

As others already noted, if rad evaluates to false (e.g. if it is 0), then the second operand is returned.

This "trick" is often used to set a default value.

Read more about logical operators.

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†: That is only 66.6% correct. The NOT operator ! will always return a boolean value.

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All the answers are correct, but they are missing an explanation of the && and || operators in JavaScript. The trick is that they don't return a boolean, they return the value where the comparison short-circuited.

For example

// Returns the first truthy value (2) since after looking at 0 and 2, we already
// know the expression is true and we don't need to evaluate the last component (3)
alert (0 || 2 || 3) 

// Returns the first falsy value (""), the comparison doesn't even
// evaluate "Hello" and "Dog"
alert( "" && "Hello" &&  "Dog" );

// No short circuiting, so the last value ("fun") is returned
alert( "string" && "fun" )

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if rad is false or 0, set ballSd.radius to 10

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Set the circle radius to either the given argument "rad" if it was given and bigger than zero otherwise to 10, which makes it the default radius.

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see this... so if the value of rad variable converted to a boolean is true, then the rad is returned, otherwise it will return 10; any variable can be converted to a boolean value: null, 0, undefined will be converted to false; not undefined will be converted to true; see implicit boolean conversions in javascript