Use every to check whether all items in an array satisfy a condition supplied by a callback. It returns true only when each item passes. section 9.1.4
Check every item
const allNinjasAreArmed = ninjas.every(ninja => "weapon" in ninja); This is false if even one ninja lacks a weapon. section 9.1.4
Listing 9.8. Testing arrays with the every and some methods
const ninjas = [
{name: "Yagyu", weapon: "shuriken"},
{name: "Yoshi" },
{name: "Kuma", weapon: "wakizashi"}
];
const allNinjasAreNamed = ninjas.every(ninja => "name" in ninja); #A
const allNinjasAreArmed = ninjas.every(ninja => "weapon" in ninja); #A
assert(allNinjasAreNamed, "Every ninja has a name");
assert(!allNinjasAreArmed, "But not every ninja is armed");
const someNinjasAreArmed = ninjas.some(ninja => "weapon" in ninja); #B
assert(someNinjasAreArmed, "But some ninjas are armed"); #B
Compare every, which checks all items, with some, which checks whether at least one item passes. section 9.1.4
