Data manipulation cheatsheet in Javascript

Use of reduce

Sum of values:


data.reduce((sum, current) => (sum + current), 0);

also can be used to create a dictionary


data.reduce((plan, current) => (plan[current.name] = current.year), {});

can also be used to extract a single value:


const value = each.contracts.reduce<number | null>((cost, plan) => {
  if (plan.type === key) cost = parseFloat(plan.cost as string);
  return cost;
}, null);


const planCost: IPlanCount = Object.values(PlanTypeEnum).reduce<IPlanCount>(
  (acc, key) => {
    acc[key] = each.contracts.reduce<number | null>((cost, plan) => {
      if (plan.type === key) cost = parseFloat(plan.cost as string);
      return 20;
    }, null);
    return acc;
  },
  {
    [PlanTypeEnum.COMMODITY_VARIABLE]: null,
    [PlanTypeEnum.VARIABLE]: null,
    [PlanTypeEnum.FIXED]: null,
    [PlanTypeEnum.NON_COMMODITY_VARIABLE]: null,
    [PlanTypeEnum.BLACK_BOX_VARIABLE]: null,
  },
);

More optimized way of doing filtering sometimes (specific case)

using reduce and list:


const countryList = data.reduce<string[]>((country, datum) => {
  if (datum.status === MeterStatus.CONNECTED && !country.includes(datum.country_code))
    country.push(datum.country_code);
  return country;
}, []);

This optimized version:

Uses filter() to select only the items with status === MeterStatus.CONNECTED.

It uses map() to extract just the country_code from each item.

Creates a Set to remove duplicates automatically.

Spreads the Set back into an array.

This approach is more efficient because:

It avoids manual checking of duplicates with includes(), which can be slow for large arrays.

It reduces the number of iterations over the data.

It leverages built-in JavaScript methods that are often optimized by the engine.


const countryList = [...new Set(
  data
    .filter(datum => datum.status === MeterStatus.CONNECTED)
    .map(datum => datum.country_code)
)];