Question

Getting single key from Value

I would like to do a backwards selection from the following JSON. I’d like to extract the abbreviation for a particular state. In this situation, the abbreviation is the key, and the value that I’m starting with is the value.

Certainly I can loop through each value, comparing the value to my value, and select the key when the match is made. Is this the best way to approach something like this? Or is there a better way?

Inverting JSON Data

Another option would to invert this data early in processing to give myself a similar set of values with the keys/values swapped. I would be interested in seeing methods for doing this efficiently as well.

var States = {AL: 'Alabama', AK: 'Alaska', AZ: 'Arizona', AR: 'Arkansas', 
              CA: 'California', CO: 'Colorado', CT: 'Connecticut', 
              DE: 'Delaware', DC: 'District of Columbia', FL: 'Florida', 
              GA: 'Georgia', HI: 'Hawaii', ID: 'Idaho', IL: 'Illinois', 
              IN: 'Indiana', IA: 'Iowa', KS: 'Kansas', KY: 'Kentucky', 
              LA: 'Louisiana', ME: 'Maine', MD: 'Maryland', MA: 'Massachusetts', 
              MI: 'Michigan', MN: 'Minnesota', MO: 'Missouri', MT: 'Montana', 
              NE: 'Nebraska', NV: 'Nevada', NH: 'New Hampshire', 
              NJ: 'New Jersey', NM: 'New Mexico', NY: 'New York', 
              NC: 'North Carolina', ND: 'North Dakota', OH: 'Ohio', 
              OK: 'Oklahoma', OR: 'Oregon', PA: 'Pennsylvania', 
              RI: 'Rhode Island', SC: 'South Carolina', 
              SD: 'South Dakota', TN: 'Tennessee', TX: 'Texas', UT: 'Utah', 
              VT: 'Vermont', VA: 'Virginia', WA: 'Washington', 
              WV: 'West Virginia', WI: 'Wisconsin', WY: 'Wyoming'};

Answer 1

There’s no “automatic” way to do this. Your only option is to loop through the list until you find the value that matches the key.

But, if you need to do this multiple times, you should have the code rebuild the JSON object with key/values swapped, so that future lookups are faster. A simple way:

function swapJsonKeyValues(input) {
    var one, output = {};
    for (one in input) {
        if (input.hasOwnProperty(one)) {
            output[input[one]] = one;
        }
    }
    return output;
}

var stateAbbrs = swapJsonKeyValues(States);