javascript - Boolean operators which return one of the operands

Question

In Python, and maybe in Javascript, the boolean or and and operators return one of the operands, instead of true or false.

• In Python, one of the operands is returned: '' || 'hello' == 'hello'
• In comparison, in PHP: '' || 'hello' == true;

Now,

• How is this behavior of boolean operators called?
• Does this also work in Javascript in all browsers?

Answer 1

As Ignacio's answer points out, these are coalescing operators. || is the null coalescing operator, && is the null-safe coalescing operator (link to follow, if I can find one sorry, I can't find a link).

They should be available in all browsers - they are both defined in the ECMA-262 1st, 2nd, 3rd and 5th editions, most current Javascript implementations are based upon 3rd or 5th. From ECMA-262 3rd edition:

The production LogicalANDExpression : LogicalANDExpression && BitwiseORExpression is evaluated as follows:
1. Evaluate LogicalANDExpression.
2. Call GetValue(Result(1)).
3. Call ToBoolean(Result(2)).
4. If Result(3) is false, return Result(2).
5. Evaluate BitwiseORExpression.
6. Call GetValue(Result(5)).
7. Return Result(6).

The production LogicalORExpression : LogicalORExpression || LogicalANDExpression is evaluated as follows:
1. Evaluate LogicalORExpression.
2. Call GetValue(Result(1)).
3. Call ToBoolean(Result(2)).
4. If Result(3) is true, return Result(2).
5. Evaluate LogicalANDExpression.
6. Call GetValue(Result(5)).
7. Return Result(6).