Why does this function return 0 [duplicate]

Question

I am trying to figure out why this javascript function returns 0 when this.position.startOffset value is 0 but not when the value is a number other than 0.

ready: function() {
    return (this.position.startOffset
            && this.position.endOffset
            && this.position.representation.trim().length >= 0
            && this.text.id
            && this.user.id
            && this.concept);
}

Looks like it's evaluating boolean, true or false, 1 or 0... is that the intention? — kvr
– kvr, Commented Aug 25, 2017 at 16:37
In JavaScript, the logical "and" operator (&&) returns the first falsy value. 0 is falsy so it will return it. — Ivar
– Ivar, Commented Aug 25, 2017 at 16:39

ibrahim mahrir · Accepted Answer · 2017-08-25 17:03:39Z

1

The && chain will stop evaluating at the first non-truthy (falsy) value and return it. Since 0 is falsy it is returned when it is encountered. If no falsy value is encountered then the last value is returned:

var a = 55 && [] && 0 && false && true;     // yeild 0 as it is the first falsy value to encounter

console.log("a:", a);

var b = 30 && true && [] && "haha";         // yeild "haha" as it is the last value (no falsy value encountered)

console.log("b:", b);

Falsy values are:

null
undefined
the empty string '' or ""
the number 0
boolean false
NaN

edited Aug 25, 2017 at 17:03

answered Aug 25, 2017 at 16:43

ibrahim mahrir

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3 Comments

KillPinguin Over a year ago

Nice answer! JavaScript is really weird.. Always would want to use typescript, or other constructs that don't rely on boolean operations on non-booleans.

Felix Kling Over a year ago

NaN is also falsy.

ibrahim mahrir Over a year ago

@FelixKling facepalm!!! Thanks.

Felipe Koji Waku · Accepted Answer · 2017-08-25 16:40:29Z

0

This happens due to how javascript handles numbers in boolean expressions. If the first parameter in a and (&&) statement returns 0, then the right parameters will not be evaluated and 0 will be returned.

answered Aug 25, 2017 at 16:40

Felipe Koji Waku

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