filter - FilterWithKeys for Javascript? -

in ramda, we're given lot of higher-order combinators practical work. 1 that's convenient .filter() utility - given array.filter() function in es6, ramda (and lodash believe) give ability filter on objects well. me, tool filter on both keys , values @ same time.

the const keyword in es6 powerful tool - using arbitrary (non-object) values, can have guarantee value never change rest of block (and deduce bugs, lots of other nice gains). other practical way filter keys , values object using for .. in, forces use mutable variable:

let xs = {...} (const k in xs) {   if (someproperty(k,xs[k])) {     delete xs[k]   } } 

i argue using filterwithkey alleviate issue:

const xs = filterwithkey(someproperty, {...}) 

is there in popular javascript library? haven't seen in ramda or lodash yet, i'm not sure else look. think suitable implementation though:

function filterwithkey (p,xs) {   r.topairs(xs).reduce((acc,x) => {     const k = x[0]     const v = x[1]     return p(k,v) ? acc[k] = v : acc   }) } 

it still requires act of rebuilding object though. there better solution?

ramda added ability filter objects on values; until natively supported lists, , otherwise delegate object's filter method if existed. in issue 1429 extended plain objects well.

there great deal of discussion in issue possibility of supporting keys parameter well, demonstrated @ odds other parts of library.

i added section ramda's cookbook describing 1 way write function yourself:

const filterwithkeys = (pred, obj) => r.pipe(   r.topairs,    r.filter(r.apply(pred)),    r.frompairs )(obj);  filterwithkeys(   (key, val) => key.length === val,    {red: 3, blue: 5, green: 5, yellow: 2} ); //=> {red: 3, green: 5} 

if have more useful short example, i'd love include instead.

like in ramda world, not mutate input data. important in functional programming, , not interested in working version built way. in other words, see feature:

it still requires act of rebuilding object though.