Binding variables should be used to pass the value into SQL statement:

-- though it cannot be used in JS UDF
CREATE OR REPLACE FUNCTION fnParseString (STRINGTOP VARCHAR,DELIMITER1 VARCHAR)
RETURNS VARCHAR NOT NULL
language javascript
AS
$$ 
var sql_A = `SELECT CASE WHEN charindex(:1, :2) = 0 THEN 1 ELSE 0 END`;
var statement = snowflake.createStatement({sqlText:sql_A,
                                           binds:[DELIMITER1,STRINGTOP]});
var rs = statement.execute();
return rs;
$$;

Alternative approach is to use SQL UDF:

CREATE OR REPLACE FUNCTION fnParseString (STRINGTOP VARCHAR,DELIMITER1 VARCHAR)
RETURNS VARCHAR NOT NULL
language sql
AS
$$ 
CASE WHEN charindex(STRINGTOP , DELIMITER1) = 0 THEN 1 ELSE 0 END;
$$;

We cannot write select statements in JavaScript UDF and need to build Stored Prod based off JS and below is same :

CREATE OR REPLACE PROCEDURE fnParseString1(STRINGTOP VARCHAR,DELIMITER1 VARCHAR)
RETURNS VARCHAR
LANGUAGE JAVASCRIPT
AS
$$
var cmd = "SELECT CASE WHEN charindex(:1, :2) = 0 THEN 1 ELSE 0 END";
var stmt = snowflake.createStatement(
          {
          sqlText: cmd,
          binds: [DELIMITER1,STRINGTOP]
          }
          );
var result1 = stmt.execute();
result1.next()
return result1.getColumnValue(1);
$$
;
call fnParseString1('this is test','');