When the micro:bit detects darkness, the servo motor springs into motion.
Continuously set the position of the Servo A to the value of the light sensor, constrained from 0 to 100 (since the light level goes up to 255).
basic.forever(function () {
    sb.setServoPosition(sb.servo(SBServo.ServoA), Math.constrain(input.lightLevel(), 0, 100))
})If using micro:bit for the first time, follow this setup.