Lots of handy code golf. Mostly ported from 140byt.es but modernized

classify.js

// add or remove classes. classify(element, '+class-to-add -class-to-remove other-class-to-add')
(a,b)=>b.split` `.map(c=>c[0]=='-'?['remove',c.slice(1)]:['add',c[0]=='+'?c.slice(1):c]).map(([d,e])=>a[d](e))

colormix.js

// make rgb colors lighter or darker, ([r, g, b], num) => [r, g, b]
(c,n)=>c.map(d=>(d+=n)<0?0:d>255?255:d|0)

escape.js

// html escape. str => str
s=>new Option(s).innerHTML

hextorgb.js

// hex=>[r,g,b]
a=>{a='0x'+a.slice(1).replace(a.length>4&&/./g,'$&$&');return[a>>16,a>>8&255,a&255]}

id.js

// random id. (len, radix) => str
(a=32,b=16)=>"0".repeat(a).replace(/./g,_=>(0|Math.random()*b).toString(b))

levenshtein.js

// levenshtein distance between strings
(a,b,c,d,e,f,g)=>{for(d=[e=0];a[e];e++)for(c=[f=0];b[++f];)g=d[f]=e?Math.min(d[--f],c[f]-(a[e-1]==b[f]),c[++f]=d[f])+1:f;return g}

localstorage.js

// localstorage helper. () => {get, set}
(a=this.localStorage||{},b=JSON)=>{get:c=>a[c]&&b.parse(a[c]),set:(c,d)=>a[c]=b.stringify(d)}

pubsub.js

// famous pubsub. ()=>{pub, sub}
(a={},b=0)=>({pub:(d,e)=>Object.keys(a).map(k=>k.split`-`[0]==d&&(a[k](e)!==!1||delete a[k])),sub:(d,e)=>a[d+--b]=e})

rgbtohex.js

// (r,g,b)=>hex
(a,b,c)=>"#"+((256+a<<8|b)<<8|c).toString(16).slice(1)

router.js

// simple spa router. ([route, fn, route2, fn2...], poll) => {}
(a,b=99,c,e=(d=location.hash)=>{for(b=0,c=d;(d=a[b++])&&!(d=d.exec(c));b++);a[b](d)})=>(e(),setInterval(e,b))

since.js

// time since (1m ago, 20s ago, 1y ago). date => str
(a,b=[1e3,60,60,24],c=0)=>{for(a=new Date-a;a>2*b[c];)a/=b[c++];return~~a+" "+"msmhd"[c]+"s ago"}

sudoku.js

// bruteforce sudoku solver...? flatArr => never (in place, throws when done)
R=(a,i,j,m,g)=>{for(i=80;a[i];i--||A);for(m=10;g=a[i]=--m;g&&R(a))for(j in a)g*=j==i||a[j]^m||i%9^j%9&&i/9^j/9&&i/27^j/27|i%9/3^j%9/3}

suffix.js

// ordinal suffix (nth, nst, ...). n => str
a=>["th","st","nd","rd"][(a=~~(a<0?-a:a)%100)>10&&a<14||(a%=10)>3?0:a]

templates.js

// simple templates. (source, ctx) => (this, ctx) => output
(a,b)=>(c,d)=>a.replace(/#{([^}]*)}/g,(a,e)=>Function("x","with(x)return "+e).call(c,d||b||{}))