Example:
Place a repeating command block with this command:
/item replace entity @e[type=zombie] armor.head with air
Active it, then run this command:
/summon zombie ~ ~ ~ {ArmorItems:[{},{},{},

]}
A zombie wearing iron helmet will appear, but if you check its data:
/data get entity <uuid of that zombie> ArmorItems
The command will return this:
Zombie has the following entity data: [{}, {}, {}, {}]