pxt-calliope/docs/reference/basic/forever.md

Forever

Keep running part of a program in the background.

basic.forever(() => {
})

Example: compass

The following example constantly checks the compass heading and updates the screen with the direction.

basic.forever(() => {
    let heading = input.compassHeading()
    if (heading < 45) {
        basic.showString("N", 100)
    } else if (heading < 135) {
        basic.showString("E", 100)
    }
    else if (heading < 225) {
        basic.showString("S", 100)
    }
    else {
        basic.showString("W", 100)
    }
})

Example: counter

The following example keeps showing the number stored in a global variable. When you press button A, the number gets bigger. You can use a program like this to count things with your @boardname@.

let num = 0
basic.forever(() => {
    basic.showNumber(num, 150)
})
input.onButtonPressed(Button.A, () => {
    num = num + 1
})

Competing for the LED screen

If different parts of a program are each trying to show something on the LED screen at the same time, you may get unexpected results. Try this on your micro:bit:

basic.forever(() => {
    basic.showNumber(6789, 150)
})
input.onButtonPressed(Button.A, () => {
    basic.showNumber(2, 150)
})

See also

while, on button pressed, in background