We use {!$Label.CustomLabelName}, when we call custom label in Visualforce page JavaScript function. But if we need to access custom labels in JavaScript file in static resource, then here is the way to get the custom label value.

Load the custom label in Visualforce Page before loading the static resource JavaScript file.

<script>
    window.$Label = window.$Label || {};
    $Label.MyCustomLable1 = '{!JSENCODE($Label.MyCustomLable1)}';
    $Label.MyCustomLable2 = '{!JSENCODE($Label.MyCustomLable2)}';
</script>

Use in JavaScript file.

console.log($Label.MyCustomLable1);
alert($Label.MyCustomLable1);