If you are loading .js from a file you have to set a variable with the csrf_token in your "main" .blade.php file where you are importing the .js and use the variable in your ajax call.
index.blade.php
...
...
<script src="{{ asset('js/anotherfile.js') }}"></script>
<script type="text/javascript">
var token = '{{ csrf_token() }}';
</script>
anotherfile.js
$.ajax({
url: 'yourUrl',
type: 'POST',
data: {
'_token': token
},
dataType: "json",
beforeSend:function(){
//do stuff
},
success: function(data) {
//do stuff
},
error: function(data) {
//do stuff
},
complete: function(){
//do stuff
}
});