Function addValueMutator

Returns a function that adds values to a map, using a hashing function to produce a key. Use addValueMutate if you don't need a reusable function.

const map = new Map(); // Create map
const mutate = addValueMutator(map, v=>v.name); // Create a mutator using default 'overwrite' policy
mutate( { name:`Bob`, size:10 }, { name: `Alice`, size: 2 }); // Add values to map
mutate( {name: `Bob`, size: 11 }); // Change the value stored under key `Bob`.
map.get(`Bob`); // { name: `Bob`, size: 11 }
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The 'collision policy' determines what to do if the key already exists. The default behaviour is to overwrite the key, just as Map.set would.

const map = new Map();
const mutate = addValueMutator(map, v=>v.name, `skip`);
mutate( { name:`Bob`,size:10 }, { name: `Alice`, size: 2 }); // Add values to map
mutate( { name:`Bob`, size: 20 }); // This value would be skipped because map already contains 'Bob'
map.get(`Bob`); // { name: `Bob`, size: 10 }
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