One-line short-circuit evaluation with && || in JavaScript

Question

var prefix = options && options.prefix || '';

In JavaScipt in my case. Can someone explain what kind of statement or condition is this? Or at the end what's the value of prefix variable?

I know about (ternary operator):

condition ? expr1 : expr2

but this was different.

Answer 1

This one-liner is the equivalent of saying:

var prefix;
if(options && options.prefix){
  prefix = options.prefix;
} else{
  prefix = '';
}

Answer 2

The statement is setting the variable "prefix" to either the value of "options.prefix", or an empty string, in such a way that if "options" does not exist, it does not throw an error.

Answer 3

It means, it options is an object, use options.prefix. If options.prefix is undefined or null or other falsy value, return empty string.

Nearly same meaning of options ? options.prefix : ''.

Answer 4

The reason this works is in Javascript logical operators evaluate to the value of the last operand.

So is options object exist, the && part of the expression will be evaulated to options.prefix.

If that is not set, the left part of the || expression will be false and the right part = the '' string - will be returned.

Answer 5

Its similar to following:

var prefix;
if(options){     // or for better understanding, its if(options != undefined)
    prefix = options.prefix;
}
else
    prefix = '';

Try the following fiddle for better understanding: http://jsfiddle.net/n41tfnh4/1/

Answer 6

Translated into human language

If variable optionsis defined with some non-falsey value

and it also has a property options.prefix whose value is also non-falsey

then set the variable prefix with this options.prefix value.

Otherwise, our prefix is set to empty string by default.

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PS. Quickly check the complete list of falsey values in JavaScript by google "javascript falsy values"

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Quicker explanation

This is a quick value check (and get) of options.prefix which is empty string by default.

(without throwing an exception when options is undefined)

Answer 7

If prefix is available on options, set it to var prefix. Otherwise, set it to an empty string.