Function unique

Tracks unique values. Returns true if value is unique. Alternatively: uniqueInstances

const t = unique();
t(`hello`); // true
t(`hello`); // false

Uses JSON.stringify to compare anything which is not a string.

Provide a custom function to convert to string to track uniqueness for more complicated objects.

const t = unique(p => p.name);
t({ name:`John`, level:2 }); // true

// Since we're judging uniques by name only
t({ name:`John`, level:3 }); // false

Return function throws an error if value is null or undefined.