how to initialize a parameter in javascript function when i define the function

Question

i have a defined a function with a parameter in javascript, and i don't know how to initialize it's parameter, any one help me. the code is bellow

<br/>Enter a number:
<input type="text" id="txt1" name="text1">
<p>Click the button to see the result</p>
<button onclick="myFunction(5)">show result</button>
<br/>
<p id="demo"></p>
<script>
  function myFunction(param) {
    alert(param);
    var z = param;
    var y = document.getElementById("txt1").value;
    var x = +y + +z;
    document.getElementById("demo").innerHTML = x;
  }
</script>

the problem which i have is: if i send an argument to the function as <button onclick="myFunction(5)">show result</button> it works, but if i don't send any argument it is not working and the result is NaN. now i want to know how i can initialize the parameter wen i define the function? as we have in php function myFunction(param=0)

Answer 1

Use param = param || 0; like

function myFunction(param) {
    param = param || 0;
    console.log(param);
    var z = param;
    var y = document.getElementById("txt1").value;
    var x = +y + +z;
    document.getElementById("demo").innerHTML = x;
  }

The above statement is saying set param to param if it is not falsey(undefined, null, false), otherwise set it to 0.

Answer 2

whenever you do not initialize a value to a parameter javascript will consider it as undefined, so you can reap the benefit of this issue and do it like:

function myFunction(param)
     {

       param = typeof param !== 'undefined' ? param : 0;

     }

hope it would be useful.