London | 26-ITP-Jan | Carlos Abreu |Sprint 2 | Data Module Groups

[carlosyabreu]

Learners, PR Template

Self checklist

• I have titled my PR with Region | Cohort | FirstName LastName | Sprint | Assignment Title
• My changes meet the requirements of the task
• I have tested my changes
• My changes follow the style guide

Changelist

PR for Sprint 2 Data Group Module

[cjyuan]

Does the following function call return the value you expect?

tally(["toString", "toString"]);

Suggestion:

• Look up an approach to create an empty object with no inherited properties.
• Use Object.hasOwn().
• Use Map object.

[carlosyabreu]

Thanks for your wise input.
It made me to rethink completely the implementation.
Going to the point.

Looking at this code, the function call tally(["toString", "toString"]) will not return the expected result of
{ "toString": 2 }. The issue is with the object property access and JavaScript's prototype chain. When the code checks if (counts[item]), it's not just checking if the object has that own property, it's checking if the value at that key is truthy.
However, for the string "toString", when counts["toString"] is accessed, JavaScript looks up the prototype chain and finds Object.prototype.toString (a function). Since functions are truthy, the condition if (counts[item]) evaluates to true even though counts doesn't have its own "toString" property.

This causes the code to try to increment counts["toString"]++, which is problematic because:

1. It's not possible to increment a function
2. This would set counts["toString"] to NaN

Using the modern approach of Object.hasOwn() implementation:

function tally(items) {
if (!Array.isArray(items)) {
throw new Error("Input must be array");
}
const counts = Object.create(null); // Empty object with no prototype

for (let i = 0; i < items.length; i++) {
const item = items[i];

if (Object.hasOwn(counts, item)) {
  counts[item]++;
} else {
  counts[item] = 1;
}

}

return counts;
}

Object.hasOwn() properly checks for own properties rather than looking up the prototype chain.

[cjyuan]

Changes look good. Well done.

[illicitonion]

Closing PR because the January ITP run has finished. Feel free to re-open if you're still working on it.