Question

Is the following function legal and portable?

function(_, _, x){
    return x;
}

Sometimes I want to write a callback that doesn’t use the leftmost parameters so I wonder what is the most concise way to do so.

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Conclusion:

function(_1, _2, x) is probably as short as it gets then.

Answer 1

It is valid in non-strict mode code, but invalid in strict mode code:

It is a SyntaxError if any Identifier value occurs more than once
within a FormalParameterList of a strict mode FunctionDeclaration or
FunctionExpression.

Source: http://es5.github.com/#x13.1

Therefore, you may want to avoid this, since at one point in the future you will want to move on to strict mode…