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).

Answer 2

It's called "coalescing". It should behave the same in any browser that claims to be compliant.

Answer 3

A recent question has been re-directed here, and although I agree that the central question has been answered accurately at a high level, part of the newer question seems to be asking about the difference between logical OR and the ternary operator in Javascript.

I would like to offer a low-level example to demonstrate the difference because I wrote most of this before the question was redirected, and the concept of a null-coalescing operator is rather abstract.

Consider these functions, which can be run in the browser console or any repl:

let logicalOrTest = function(val) { 
  let a = 2;
  let b = 3;
  console.log(val === a);
  console.log(val === b);
  console.log(val === (a || b));
  console.log(val !== (a || b));
}

Expect logicalOrTest(2) to log true false true false

Expect logicalOrTest(3) to log false true false true

Expect logicalOrTest(4) to log false false false true

This reveals that comparing any value val to (a || b) will return true when the value is equivalent to the first condition, and false for any other value, including the second.

To put it simply, if condition a is met, condition b is not even considered. On the other hand, if condition a is not met, b will be considered before determining what to do.

Now let's look at the ternary operator:

let ternaryTest = function(val) { 
  let a = 2;
  let b = 3;
  val === a ? console.log(true) : console.log(false);
  val === b ? console.log(true) : console.log(false);
  val === (a || b) ? console.log(true) : console.log(false);
  val !== (a || b) ? console.log('neither condition was met') : console.log('wat');
}

Expect ternaryTest(2) to log true false true "wat"

Expect ternaryTest(3) to log false true false "neither condition was met"

Expect ternaryTest(4) to log false false false "neither condition was met"

Like logical OR (||), the ternary operator x ? x : y will pass any falsy value into the scope of the second expression, which also includes values equivalent to the second condition. If condition a is met, condition b is not considered.

The difference is that if condition a is not met in a ternary operation, expression a will be ignored and expression b will always be executed.

So, while they can be used for similar purposes, it's important to understand which one is more efficient--especially if you are triggering large blocks of code within the scope of your operation.