Group arbitrary labels without confusing them with object properties

Trace the fixed program

Count the three fixed labels markup, toString and markup. Keep groups in the order their keys first appear.

const labels=['markup','toString','markup'];const counts=new Map();
for(const label of labels)counts.set(label,(counts.get(label)??0)+1);
console.log(JSON.stringify([...counts]));

[["markup",2],["toString",1]]

Explain the result

Map.get reads stored entries rather than prototype properties. Missing keys yield undefined; ?? supplies zero before the increment. Updating markup does not move its insertion position. Ordinary objects need an explicit own-property or null-prototype strategy instead.

Test the alternative

On an ordinary empty object, toString is inherited and is a function, not a missing counter. Checking its type proves the collision without relying on an engine-specific function string.

const counts={};const value=counts['toString']??0;
console.log(JSON.stringify({own:Object.hasOwn(counts,'toString'),type:typeof value,zero:value===0}));

{"own":false,"type":"function","zero":false}

Work the boundary case

const counts=new Map([[1,'number'],['1','text']]);
console.log(JSON.stringify({size:counts.size,number:counts.get(1),text:counts.get('1')}));

{"size":2,"number":"number","text":"text"}