Something unintelligible in javascript

Question

What does it mean?

1.

a==b&&b={}

2.

a==b||b={}

I didn't find answer

Look up "javascript operators" for a start. There are many duplicates. There is some "magic" here because of the *short circuit* nature of `&&` and `||`. — , Nov 04 '12 at 00:02
Both yield `ReferenceError: Invalid left-hand side in assignment` — Michael Berkowski, Nov 04 '12 at 00:02
http://stackoverflow.com/questions/3737139/reference-what-does-this-symbol-mean-in-php — inhan, Nov 04 '12 at 00:03
@pst Even if it is a variable in scope, it throws the ReferenceError. — Michael Berkowski, Nov 04 '12 at 00:04
@Esailija I'd say he needs to *avoid* this confusing pattern. — Šime Vidas, Nov 04 '12 at 00:07
@user1758424 Where did you stumble upon that pattern? You "need" parens because logical operators (e.g. `&&`) have precedence over assignments, so in `a==b&&b={}`, `{}` is assigned to `a==b&&b`. — Šime Vidas, Nov 04 '12 at 00:25

score 1 · Answer 1 · answered Nov 04 '12 at 00:03

1

It means, if a equals b, then set b to an empty object. The second one means more or less the same. The difference is that the first one will not set b to an empty object if a and b are not equal. But the second one will always do that no matter what. That is because the OR operator in javascript continues where as the AND operator will short-circuit.

http://www.openjs.com/articles/syntax/short_circuit_operators.php

answered Nov 04 '12 at 00:03

Mohamed Nuur

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It only works when you put parenthesis around the assignment portion, otherwise due to the order of operations, it will fail. – Mohamed Nuur Nov 04 '12 at 00:05

score 1 · Accepted Answer · answered Nov 04 '12 at 00:04

1

a==b is the condition to be tested. The operators && and || test the condition very much like a ternary operator but you use it when there's only one condition you need to test, either false || or true &&. It would be the same as:

if ( a == b ) { b = {} } // a == b && ( b = {} )
if ( a != b ) { b = {} } // a == b || ( b = {} )

But as Esailija pointed out in the comments, seems like you're missing some parenthesis:

a == b || ( b = {} )

answered Nov 04 '12 at 00:04

elclanrs

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Why I need to add parenthesis? And what is so bad in my question? – user1758424 Nov 04 '12 at 00:17
The parenthesis are needed because of the [operator precedence](http://www.scriptingmaster.com/javascript/operator-precedence.asp). Parenthesis are _always_ evaluated first since they have highest precedence. The assignment operator `=` has lower precedence than the or operator `||`. – elclanrs Nov 04 '12 at 00:24

Something unintelligible in javascript

2 Answers2