Plural form of "walk-in"

Question

I looked up the definition of "walk-in" as a noun in Merriam-Webster dictionary,

walk-in (noun)

1. a walk-in refrigerator or cold storage room
2. an easy election victory
3. a person who walks in without an appointment

and wonder how to spell the plural form of "walk-in" (definition 3), walks-in or walk-ins? I know a word of a similar morphological structure and its plural form is "passers-by."

Answer 1

With nouns that are made up of noun + preposition, you pluralise the noun, rather than the whole word, giving passers-by or passersby. Other examples of this are runners-up and hangers-on.

With nouns that are made up of verb + preposition, you cannot pluralise a verb, so the plural is made by adding an s to the word as a whole. We therefore have walk-ins, phone-ins, strikeouts, lean-tos, set-tos, take-aways, carry-ons etc.