JSLint - Do not mutate parameter <x> when using 'arguments'?

Question

All,

I use JSLint to validate my JS files. In my most recent project, I am using the following format to set default values for a number of JavaScript functions (further detailed here):

function(a, b, option) {
    option = arguments.length > 2 ? option : "some default value";
    // ...
}

This however causes the latest build of JSLint to produce the following error:

"Do not mutate parameter 'option' when using 'arguments'."

I am aware that using the more common method for assigning defaults (i.e., option = option || {};) supresses the error; however, this will produce incorrect behaviour if I intend to pass a falsey value to option.

Is the only solution to this issue to introduce a new variable? e.g.:

var option2 = arguments.length > 2 ? option : "some default value";

Answer 1

I guess JSLint warns you since you try to modify one of the input arguments by using a check with the arguments keyword. JSHint, however, does give me any warning when trying your code.

A solution to your problem would be to check if option is defined or not, that way you go around the problem with sending in falsy values:

function(a, b, option) {
    if(typeof option === "undefined") {
        option = "some default value"; 
    }
    // ...
}

If you find that it is cumbersome to write this typeof check everytime, create an isDef function:

function isDef(param) {
    return typeof param !== "undefined";
}
function(a, b, option) {
    option = isDef(option) ? option : "some default value"; 
    // ...
}

// Simon A.