Variable equals to A || B. How does this work?

Question

I came across this code:

function extend(out){
    out = out || {};
    for (var i = 1; i < arguments.length; i++){
        var obj = arguments[i];
        if (!obj) continue;
        for (var key in obj){
            if (obj.hasOwnProperty(key))
                out[key] = (typeof obj[key] === 'object') ? extend(out[key], obj[key]) : obj[key];
        }
    }
    return out;
}

What does the line out = out || {}; mean and how does it work?

This function bascially combines different objects into one object.

This http://stackoverflow.com/q/4446433/6647153 is exactly your question! — ibrahim mahrir, Feb 22 '17 at 02:53

score 1 · Accepted Answer · answered Feb 22 '17 at 02:55

It sets a default value for out, ie if out is not provided to the function, (or it is a falsy value such as false, 0, null, undefined or ''), then out is assigned {}.

How it works: Javascript uses short circuit evaluation, which means the the or (||) operator will return the first non-falsy value. If out is not falsy, nothing changes, if out is falsy, it gets set to {}.

score 1 · Answer 2 · answered Feb 22 '17 at 02:56

In Javascript, the || operator returns the first operand if it is a truthy value; otherwise, it returns the second. So in context here, if out is a falsy value (such as undefined), then out = out || {} will assign {} to out; otherwise, it will assign the current value of out into out, which effectively does nothing.

The intent is to provide a default value for out. If the caller passes false, null, or undefined as the first argument, then out will be a new object. If the caller passes an object as the first argument, then the function will modify that object.

score 0 · Answer 3 · edited May 23 '17 at 12:09

0

It is basically the short hand of doing this

if(!out){
    out = {};
}

Essentially if out does not evaluate to 'true' then out will be set to an empty array in this case.

Already answered here also: What does the construct x = x || y mean?

edited May 23 '17 at 12:09

Community

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answered Feb 22 '17 at 02:54

Aidan

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That's a empty object, not an array. – Paul Feb 22 '17 at 02:56
Sorry, you're correct, I've been working in C# all week. – Aidan Feb 22 '17 at 02:58

score 0 · Answer 4 · answered Feb 22 '17 at 02:56

0

That is a simplified form of this:

function extend(out){
    if (!out) {
        out = {};
    }
    ...
}

answered Feb 22 '17 at 02:56

Teocci

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score 0 · Answer 5 · edited May 23 '17 at 12:00

0

out = out || {};

If the variable 'out' is not set prior to this line, the value will be set to an empty array. JavaScript OR (||) variable assignment explanation

edited May 23 '17 at 12:00

Community

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answered Feb 22 '17 at 02:56

L L

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Variable equals to A || B. How does this work?

5 Answers5