Reverse lookup object with array

Question

Let say if I have an object like this

resourceMap = {
    "a": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
    "b": [11, 12],
    "c": [21, 23],
    "d": [54, 55, 56, 57, 510]
};

What is the best way to figure out if resourceId = 21 would be "c"?

We don't know the key names or number of keys. It only matches once: meaning 21 will belong to only one key "c".

I am thinking of looping through all keys and do indexOf(), but I don't feel it's "elegant" enough.

I could use Underscore but try to avoid and go with what Angular or jQuery or just vanilla Javascript.

Answer 1

It's perfectly acceptable to have numeric property names for objects in JavaScript. We can use this to our advantage to build a second object that maps everything in reverse. This will make lookups inexpensive.

var reverseMap = {};
for(var propName in resourceMap)
{
    var numsArr = resourceMap[propName];
    numsArr.forEach(function(num){
        reverseMap[num]=propName;
    });
}
console.log(reverseMap[54]); //'d'

http://jsfiddle.net/y11sbgbv/

Building the reverseMap can also be done more "functionally" (e.g. without using side-effects) as follows:

var reverseMap2 = Object.keys(resourceMap).reduce((acc, propName) =>          
  resourceMap[propName].reduce((a, num) => {
    a[num] = propName;
    return a;
  }, acc), {});

Answer 2

Preprocess into a different look up table

var revMap = []; // or var revMap = {};
Object.keys(resourceMap).forEach(function(key) { 
    resourceMap[key].forEach( function (ind) {
         revMap[ind] = key;   
    });
});
console.log(revMap[21])

or look up on demand:

resourceMap = { "a": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10], "b": [11, 12], "c": [21, 23], "d": [54, 55, 56, 57, 510] };
function findMapKey (map, ind) {
    var match = null;
    Object.keys(map).some(function(key) {
        var test = map[key].indexOf(ind)!==-1;
        if (test) { 
            match = key; 
        } 
        return test;
    });
    return match;
}
console.log(findMapKey(resourceMap, 21));