Count by key in JavaScript

Tallies how many items fall under each computed key.

function countBy(items, keyFn) {
  const counts = {};
  for (const item of items) {
    const key = keyFn(item);
    counts[key] = (counts[key] || 0) + 1;
  }
  return counts;
}

Keywords and builtins used here

const
for
function
of
return

The run, in numbers

Lines: 8
Characters to type: 161
Tokens: 51
Three-star pace: 95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 32 seconds.

Type this snippet

Step 6 of 8 in Objects & collections, step 32 of 43 in Language basics.

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