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||—expr1 || expr2(Logical OR)Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false..
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&&—expr1 && expr2(Logical AND)Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.
All values in Javascript are either “truthy” or “falsy”.
The following values are equivalent to false in conditional statements:
- false
- null
- undefined
- The empty string
""(\'') - The number 0
- The number NaN
All other values are equivalent to true.
So… var test = 0 || -1 ; returns -1.
If it was var test = 0 || false || undefined || "" || 2 || -1 it would return 2
Logical operator on MDN