[Vuejs]-Unexpected Token : (colon) in Vue Typescript file

Make sure that you specified TypeScript in the script tag

<script lang="ts">
    ...
</script>

default is a preserved keyword in vue, try with something else., even in js it should be used inside switch case to return something or for exporting data

Wrap default in quotes:

@Prop({ 'default': 'default ToDo' })