Javascript Question Mark & Double Pipes

Question

I am new to javascript and so far it is my understanding that:

? & : is used for "if true, do this, if false do this"

However, I am having a little more trouble with ||. From my browsing it seems something like "if the first one is true do that, otherwise do this"

I am trying to figure out the following code - any suggestions on what they mean together in this context?:

function isSubset(series, description){
    var subset = true;
    var exactMatch = true;
    demoCodes = ['age', 'edu', 'race', 'sex'];
    for (var i = 0; i < demoCodes.length; i++){
        var demoCode = demoCodes[i];
        subset = (subset) ? (description[demoCode] == 0 || description[demoCode] == series[demoCode]) : false;
        exactMatch = (exactMatch) ? description[demoCode] == series[demoCode] : false;
    }
    return {subset: subset, exactMatch: exactMatch};
}

Thanks! Cheers

MDN Documentation for [Ternary Operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_Operator) and [Logical OR](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators) — epascarello, Oct 29 '14 at 17:15
` if (subset) { if (description[demoCode] == 0) { subset = true; } else { subset = (description[demoCode] == series[demoCode]); } } else { subset = false; }` — epascarello, Oct 29 '14 at 17:22
You might find this [analysis of `||` vs `&&` vs `??`](https://stackoverflow.com/a/70143185/8910547) useful. — Inigo, Nov 28 '21 at 12:06

foxygen · Accepted Answer · 2014-10-29T17:22:31.743

|| means "or". The left side of the || is evaluated first. If it resolves to true, then the expression resolves to true. If, on the other hand, the left side of the || operator resolves to false, then the right side will be evaluated and returned.

Example 1:

1 == 1 || 1 == 0

Will evaluate to true, since the left side of the || operator is true.

Example 2:

1 == 2 || 1 == 1

The left side resolves to false, so the right side is evaluated and returned. In this case, 1==1 so the whole expression (1 == 2 || 1 == 1) resolves to true.

Example 3:

1 == 2 || 1 == 3

The left side resolves to false, so the right side is evaluated and returned. In this case, 1 does not equal 3, so the whole expression (1 == 2 || 1 == 3) resolves to false.

To put it more simply, if either of the expressions "held together" by the || operator are true, then the expression will return true. Otherwise, it will return false.

ah, while your original answer made sense, the added edit with the bold clarification really helped me understand the logic in this function. thanks! — As3adTintin, Oct 29 '14 at 17:28

score 1 · Answer 2 · answered Oct 29 '14 at 17:19

subset = (subset) ? (description[demoCode] == 0 || description[demoCode] == series[demoCode]) : false;

is equal to

if(subset){
  subset = (description[demoCode] == 0 || description[demoCode] == series[demoCode);
}
else { subset = false; }

The || is an or operator here and evaluates to true or false

Javascript Question Mark & Double Pipes

2 Answers2

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