Default parameters

Why default.parameters ?

The default parameters of the JavaScript functions are undefined. If we do not enter a parameter, it is equal to “undefined“. As you can see in the previous lessons, javascript functions can take many identical or different parameters. If we do not enter these parameters, the “undefined” value returns as I said above.

For example; We have a 2-parameter function. In this function, the first parameter is x, the second parameter name is y. Let’s define this function as follows.

        let sum = function (x , y) {
            return x + y
        }

If we call this function as follows, we have no problems:

        console.log(sum(3, 6)); // 9

If we don’t even enter a parameter. We encounter this problem:

       console.log(sum( 6)); // NaN

We can solve this problem as follows:

        function sum(x, y) {
            if (y === undefined) {
                y = 0;
            }
            return x + y;
        }

        console.log( sum(3) )

output:

3

or;

    function sum(x, y=2) {
        return x + y;
    }

    console.log( sum(3) )

output:

5

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JavaScript Default Parameters

Example 1

        function myFunction(example = "example"){
            console.log(typeof example);
        }

            myFunction();
            myFunction(undefined);
            myFunction("");
            myFunction(null);

output:

string
string
string
object

---

Example 2

        function arrayFunction(arr, list=[]){
            list.push(arr);
            return list;
        }

            console.log(arrayFunction(5))
            console.log(arrayFunction(3))

output:

[5]
[3]

---

Browser Support

Chrome	49
Edge	15
Firefox	14
Internet Explorer	no
Opera	36
Safari	10
Android webview	49
Chrome for Android	49
Edge mobile	14
Firefox for Android	15
Opera Android	36