How to pass the value 'undefined' to a function with multiple parameters?

I want to pass the value of 'undefined' on a multiple parameter function but without omitting the parameter.

What do I mean with "without omitting the parameter". I mean that we should not just omit the parm2 like this example:

function myFunction (parm1, parm2) {}
myFunction("abc");

This will indeed make parm2 undefined, but I am not allowed to do it this way because I will need to specify other parameters AFTER the omitted parameter, so the previous method won't work in the case I want to make parm1 undefined BUT also want to have other parameters after this one to hold a value.

I have tried solving the problem with:

myFunction( ,"abc"); //doesn't seem to work

Update:

and myFunction(undefined, "abc"); « this reliably works now.

However, it is worth mentioning that:

Setting a variable to undefined is considered a bad practice, we should be using null instead.

xdazz

myFunction(undefined,"abc"); this way should work, what is the problem?

see here

Here is undefined documentation from mozilla, supported by all browsers

The void operator seems to be the most common way to explicitly get undefined.

You would use it like this in your example:

myFunction(void 0, "abc");

It's also a reliable method for comparing against undefined that is guarded against undefined being accidentally overridden in older JavaScript environments:

var x;
if (x === void 0) {
    // this will execute
}

dbrin

A better approach might be passing Object with named attributes and then look for those specific attribute values. This way you don't have to be dependent on number of arguments.
Example: Object.dummy

I just had an idea and it seems to work:

var undf;

myFunction(undf, "abc");

I am sure there are better ways, however I post this

I think the closest you'll get to this is passing null as a parameter. It's not undefined, but for most cases it's close enough.

Try to use this method if you plan on adding an indefinite amount of parameters:

function myFunc(params) {
    // Define default values
    var name = 'John';
    var age = '40';
    // You can loop through them
    for (var p in params) {
        alert(p + ':' + params[p]);
    }
    // And read them in like this
    if (typeof params.name != 'undefined') {
        name = params.name;
    }
    if (typeof params.age != 'undefined') {
        age = params.age;
    }
    alert(name + ' ' + age);
}

alert('test1');
myFunc({name:'Bob', age:'30'});
alert('test2');
myFunc({name:'Bob'});

You can use apply and an array of parameters to pass "undefined" as one of the parameters. For example, you wanted to pass parm1 as "undefined":

function myFunction (parm1, parm2) {

    if(typeof (parm1) === "undefined"){
        alert("parm1 is undefined")
    }

    if(typeof (parm2) === "undefined"){
        alert("parm2 is undefined")
    }

}

var myParameters = [undefined, "abc"];

myFunction.apply(valueForThis, myParameters );

You need to handle the function call with undefined variable in the function code itself in one of the following way :-

 function myFunction (parm1, parm2) {

         if (parm1 !== undefined) {
             // execute code if 1st param is undefined
        }

         if (parm2 !== undefined) {
             // execute code if 2 param is undefined
        }

        // execute other part of code

    }

Now you can call the above function in following ways:-

myFunction(undefined,"abc"); // with 1st param value not known

myFunction("cde",undefined); // with 2nd param value not known

myFunction("cde","abc");  // with both param value known

myFunction(undefined,"abc") should absolutely work, unless someone cruel has redefined undefined! If you want to be safe, there are dozens of ways to get the undefined value which avoid the assumption that the thing called "undefined" does in fact have the special undefined value:

void 0
var undef; undef
[][0]
{}.foo
// `undef` is guaranteed to have the undefined value within this closure
(function(undef){ undef }())
// this expression evaluates as undefined
(function(){}())

void 0 is the most widely used (compiled CoffeeScript includes this wherever the undefined value is required).

If its possible , could you rearrange the function as myFunction (parm2, parm1)

ie. try to ensure possible undefined parameters are last in the function , this might not work if more parameters are introduced.

An easy way of doing this, when acceptable is to pass undefined. But better yet as follows per W3C

Javascript-missing arguments

function myFunction(x, y) {
if (y === undefined) {
      y = 0;
    } 
}

Just to give an examples on what @dbrin was explaining, and @alphapilgrim might have wished to check.

simpleFunction({params1:12, params2:"abc"}); //use expected arguments
simpleFunction({params2:"abc"}); //use only one
simpleFunction({params2:"abc", params1:12, iDoNotExist:"I will not be included"}); //use expected arguments in any order
simpleFunction(); //ignore arguments and uses defaults
simpleFunction(2123); //ignores non object arguments 

function simpleFunction(someParams){
  var myParams = {
    params1:null,
    params2:"cde" //default value for second parameter
  };
  simpleExtend(myParams, someParams);
  
  console.log(myParams);

}


//to include in your utilities
function simpleExtend(currentParams, newParams){
  
  if(typeof currentParams !== "undefined" && typeof newParams !== "undefined"){
    Object.keys(newParams).forEach(function(key){
   
      if(typeof currentParams[key] !== "undefined"){
        currentParams[key] = newParams[key];    
      }

    });
 
  
  }
  
 

}

来源：https://stackoverflow.com/questions/8998612/how-to-pass-the-value-undefined-to-a-function-with-multiple-parameters

标签

javascript

function

parameter-passing

undefined