How to create optional arguments for a SASS mixin?

To create optional arguments for a SASS @mixin, the mixin declared must be passed and it should be included.

The following approach will explain clearly.
Syntax:

@mixin function-name($var1, $x:val){

/* stylesheet properties */
}
@include function-name(value, 0);

Approach: Assigning Null in @mixin includes

• We can make an argument of mixin optional by defining its default value that argument isn’t passed by @include.
• Passing null or 0 or no argument passed by @include of mixin led to Optional Arguments.
• Optional arguments has variable name followed by colon and a and a SassScript expression

Example 1: Below example illustrates above approach.
SASS file: style.scss

/* Here $x:5px Optional Arguments*/
@mixin shadowbox($hoff, $voff, $blur, $spread, $color, $x:5px){
    -webkit-box-shadow: $hoff $voff $blur $spread $color;
    -moz-box-shadow: $hoff $voff $blur $spread $color;
    box-shadow: $hoff $voff $blur $spread $color;
    border: $x solid $color;
/* null is passed in mixin @include*/
    div{
    @include shadowbox(0, 8px, 6px, -6px, black, null);
        display: block;
    }

Compiled CSS file:style.css

div{
    -webkit-box-shadow: 0 8px 6px -6px black;
    box-shadow: 0 8px 6px -6px black;
    border: 5px solid black;
    display: block;
  }

Example 2: Below example illustrates above approach.
SASS file: style.scss

/* Here $attach:fixed Optional Arguments*/
@mixin backgroundstretch( $bgsize, $attach:fixed ){
        background-attachment: $attach;
        background-position: center;
        -webkit-background-size: $bgsize;
        -moz-background-size: $bgsize;
        -o-background-size: $bgsize;
        background-size: $bgsize;
}
body {
        color:$grey;
    font-family: Helvetica, sans-serif;
    background-image: url('/img/backimg.jpg');
    background-repeat: no-repeat;
    /* 0 is passed in mixin @include*/
    @include backgroundstretch(cover, 0);

}

Compiled CSS file:style.css

body {
  color: #919191;
  font-family: Helvetica, sans-serif;
  background-image: url("/img/backimg.jpg");
  background-repeat: no-repeat;
  background-attachment: fixed;
  background-position: center;
  background-size: cover;
}

Reference: https://sass-lang.com/documentation/at-rules/mixin#optional-arguments