>> unitless(100)

true

>> unitless(100px)

false

>> unitless(100em)

false

>> unitless(100%)

false

>> unitless(1 /2 )

true

>> unitless(1 /2 + 2 )

true

>> unitless(1px /2 + 2 )

false

// 判断如果没有单位则加上单位"px"
@mixin adjust-location($x, $y) {

  @if unitless($x) {

    $x: 1px * $x;

  }

  @if unitless($y) {

    $y: 1px * $y;

  }

  position: relative;

  left: $x;

  top: $y;

}

.botton{

    @include adjust-location(20px, 30);

}