Javascript default array/object parameter value

Question

I'm making up a way of setting function array parameter to a default value (one ES5 not new ES6/2015).

My first idea was this:

window.fnc = function(args){
    defaults = {
        height: 120,
        width: 32
    }

    args = args || defaults;

    console.log(args['height']+'x'+args['width'])

}

and will be used like this:

fnc({height:45, width:12}); //Result: 45x12

or

fnc(); //Result: 120x32

This works ok. But of course at line #7 the args is replaced by defaults or it is not, so if either one of the 2 values are missing i get an undefined.

fnc({height:45}); //Result: 45xundefined

How do I go about this?

Sebass van Boxel · Accepted Answer · 2015-12-30 23:55:25Z

3

I use this code snippet from time to time:

 args = args || defaults;
     for (var opt in defaults) {
        if (defaults.hasOwnProperty(opt) && !args.hasOwnProperty(opt)) {
            args[opt] = defaults[opt];
        }
    }

Basically you iterate over all the values from the default arguments (for (var opt...) and check against the values of the arguments whether it is missing or not. (if (defaults.has...). If it is missing, add the default value (args[opt] =...).

(I think I even found this snippet on SA, but not sure. Will add a link to the original answer if so.)

answered Dec 30, 2015 at 23:55

Sebass van Boxel

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Comments

anotherdev · Accepted Answer · 2015-12-30 23:57:48Z

2

You can iterate the keys of default to copy the one missing in args.

window.fnc = function(args){
    defaults = {
        height: 120,
        width: 32
    }

    //args = args || defaults;
    if(args == null)
      args = defaults;
    else {
      for(var i in defaults) {
        if(args[i] == null) args[i] = defaults[i];
      }
    }

    console.log(args['height']+'x'+args['width'])

}

If you have sub-objects in defaults, you will need to do a recursive function.

EDIT:

Recursive example:

// check if obj is an object
function isObject(obj) {
  return typeof obj == "object";
}
// check if obj is an array
function isArray(obj) {
  return (typeof obj == "object") && (Object.getPrototypeOf( obj ) === Array.prototype);
}
// copy the attributes from original into dest
function copy_attributes(original, dest) {

  for(var i in original) {

    if(isArray(original[i])) {

      dest[i] = JSON.parse(JSON.stringify(original[i])); // copy
    }
    else if(isObject(original[i])) {

      copy_attributes(original[i], dest[i]);
    }
    else {

      dest[i] = JSON.parse(JSON.stringify(original[i])); // copy
    }
  }
};

edited Dec 30, 2015 at 23:57

answered Dec 30, 2015 at 23:50

anotherdev

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3 Comments

Mr.Web Over a year ago

Ok, what do you mean with the recursive? any example?

anotherdev Over a year ago

args = args || defaults; sets args to args if args is not false, else it sets args to defaults.

anotherdev Over a year ago

@Mr.Web: the function copy_attributes should help if your defaults variable contains sub-elements.

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