I have a TSL257LF with GND, 3.3V, and output connected to A0 of my arduino. The test sketch is loop() with Serial.println(analogRead(A0)) that in Serial Monitor shows numbers 674 to 677 oblivious to the sensor being illuminated or in darkness.
I tried another TSL257LF, putting a 10k pull-up resistor on output, and using 5VDC all to no avail.
I haven't added a decoupling cap (.1uF) to the supply lines yet, but I don't think that should be such a factor.
I'm just trying to quickly detect transitions from light to dark and the TSL257 should be faster than any LDR.