Quoting ECMA-262 First Edition (June 1997), Chapter 11, Section 11 (Binary logical operators)[1]:

    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).

That is, the behavior has always been "If the first expression is false-ish, return the first expression, otherwise return the second expression" ("BitwiseORExpression" is a class of expressions that include a lot of things, including equality operators).

JavaScript does not have any operators that is not explicitly listed in a version of ECMA-262. It would be correct to refer to the construct as a "guard", but incorrect to refer to it as a "guard operator". Calling it "guard operator" also does not promote an understanding of the underlying construct.

1: https://www.ecma-international.org/publications/files/ECMA-S...

> the behavior has always been

The ECMA spec only defines Javascript 1.3 and above. See this description for logical operators for Javascript 1.1: https://web.archive.org/web/20060318153542/wp.netscape.com/e...

ECMA-262 defines ECMAScript. "JavaScript 1.3" refers to a Netscape-specific language implementation ("Mocha", "LiveScript", "JavaScript"), with version 1.3 being based partially based on ECMA-262 Second Edition. Some of it might still live on in Firefox, but it is certainly not what people refer to as "JavaScript".

Doing some research, it appears that Netscape changed the logical operator behavior in version 1.2 (https://web.archive.org/web/19981202065738if_/http://develop...).

However, they did not highlight this change at all (https://web.archive.org/web/19970630092641fw_/http://develop...), so I suspect it was just a minor cleanup, potentially related to the equality operator change.

As a side-note: Netscape scripting was awful. This was how you casted an object to a Number:

    Number(x) = x;