innerJoin(dfToJoin, columnNames) {
        return this._join(dfToJoin, columnNames, ["in"]);
    }
Join two DataFrames with inner mode.
@param {DataFrame} dfToJoin The DataFrame to join.
@param {String | Array} columnNames The selected columns for the join.
@returns {DataFrame} The joined DataFrame.
@example
df.innerJoin(df2, 'id')
df.join(df2, 'id')
df.join(df2, 'id', 'inner')