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const fillMissingHours = hours => {
  const result = [];
  let last;

  for (const entry of hours) {
    for (let i = last + 1; i < entry.hour; i++) {
      result.push({hour: i, value: 0});
    }

    result.push({...entry});
    last = entry.hour;
  }

  return result;
};

const rawData = [
  {hour: 3, value: 3},
  {hour: 5, value: 9},
  {hour: 10, value: 5},
];
console.log(fillMissingHours(rawData));

• Use standard formatting with Prettier (avoid smushed together expressions like i+currentHour+1 and extra spacing like {hour: 10, value: 5} ,).

• Avoid using the type in the variable name, so iString should be named what it is only. i generally means "index" so I'd avoid that in the name too.

• Don't use in to iterate arrays because it has gotchas you can read about in the docs. Prefer for ... of.

• Compute the final value once when possible. Consider these lines:

  const gap = nextAvailableHour - currentHour
  const emptyHours = Array(gap-1)
  

  It's great that you've broken out a variable gap, but since it's only used this one time, I'd compute its final value up front:

  const gap = nextAvailableHour - currentHour - 1
  const emptyHours = Array(gap)
  

  This is a subtle distinction, but it's good practice to keep things in one place and be transparent about variables. Otherwise, the code reads like "The gap is nextAvailableHour - currentHour. Just kidding, it's actually - 1 that thing I just wrote on the previous line.".

• Write a test suite to validate the logic thoroughly. This is the sort of function that can have a lot of edge cases. I was lazy and didn't test this, so if there's a bug, that only proves my point.

This actually isn't too bad, but you can probably see why I went for simpler syntax.

/**
 * @typedef {Object} HoursData - Represents an entry containing hour-value data.
 * @property {number} hour - The hour of the entry.
 * @property {number} value - The value associated with the hour.
 */

/**
 * Fills missing hours in a series of hour-value entries.
 * @param {HoursData[]} hours - A series of hour-value entries sorted ascending by hour.
 * @returns {HoursData[]} - A copy of the hours with gaps filled.
 */
const fillMissingHours = hours => {
  const result = [];
  let last;

  for (const entry of hours) {
    for (let i = last + 1; i < entry.hour; i++) {
      result.push({hour: i, value: 0});
    }

    result.push({...entry});
    last = entry.hour;
  }

  return result;
};

const rawData = [
  {hour: 3, value: 3},
  {hour: 5, value: 9},
  {hour: 10, value: 5},
];
console.log(fillMissingHours(rawData));

I used JSDoc instead of TS so this is runnable in the browser, but I assume you have the TS types handled already.

• Use standard formatting with Prettier (avoid smushed together expressions like i+currentHour+1 and extra spacing like {hour: 10, value: 5} ,).

• Avoid using the type in the variable name, so iString should be named what it is only. i generally means "index" so I'd avoid that in the name too.

• Your code only works when the input is pre-sorted. That's fine but needs to be documented as an invariant.

• Don't use in to iterate arrays because it has gotchas you can read about in the docs. Prefer for ... of.

• Compute the final value once when possible. Consider these lines:

  const gap = nextAvailableHour - currentHour
  const emptyHours = Array(gap-1)
  

  It's great that you've broken out a variable gap, but since it's only used this one time, I'd compute its final value up front:

  const gap = nextAvailableHour - currentHour - 1
  const emptyHours = Array(gap)
  

  This is a subtle distinction, but it's good practice to keep things in one place and be transparent about variables. Otherwise, the code reads like "The gap is nextAvailableHour - currentHour. Just kidding, it's actually - 1 that thing I just wrote on the previous line.".

• Write a test suite to validate the logic thoroughly. This is the sort of function that can have a lot of edge cases. I was lazy and didn't test this, so if there's a bug, that only proves my point.

This actually isn't too bad, but you can probably see why I went with the simpler syntax in my recommendation.

• Use standard formatting with Prettier (avoid smushed together expressions like i+currentHour+1 and strange spacing like {hour: 10, value: 5} ,).
• Avoid using the type in the variable name, so iString should be named what it is only. i generally means "index" so I'd avoid that in the name too.
• Don't use in to iterate arrays because it has gotchas you can read about in the docs. Prefer for ... of.
• Write a test suite to validate the logic thoroughly. This is the sort of function that can have a lot of edge cases. I was lazy and didn't test this, so if there's a bug, that only proves my point.

• Use standard formatting with Prettier (avoid smushed together expressions like i+currentHour+1 and extra spacing like {hour: 10, value: 5} ,).

• Avoid using the type in the variable name, so iString should be named what it is only. i generally means "index" so I'd avoid that in the name too.

• Don't use in to iterate arrays because it has gotchas you can read about in the docs. Prefer for ... of.

• Compute the final value once when possible. Consider these lines:

  const gap = nextAvailableHour - currentHour
  const emptyHours = Array(gap-1)
  

  It's great that you've broken out a variable gap, but since it's only used this one time, I'd compute its final value up front:

  const gap = nextAvailableHour - currentHour - 1
  const emptyHours = Array(gap)
  

  This is a subtle distinction, but it's good practice to keep things in one place and be transparent about variables. Otherwise, the code reads like "The gap is nextAvailableHour - currentHour. Just kidding, it's actually - 1 that thing I just wrote on the previous line.".

• Write a test suite to validate the logic thoroughly. This is the sort of function that can have a lot of edge cases. I was lazy and didn't test this, so if there's a bug, that only proves my point.

const rawData = [
  {hour: 3, value: 3},
  {hour: 5, value: 9},
  {hour: 10, value: 5},
];

const fillMissingHours = hours => {
  const result = [];
  let last;

  for (const entry of hours) {
    for (let i = last + 1; i < entry.hour; i++) {
      result.push({hour: i, value: 0});
    }

    result.push({...entry});
    last = entry.hour;
  }

  return result;
};

console.log(fillMissingHours(rawData));

I'll admit, this is a wee bit clever because I'm relying on the fact that undefined + 1 is NaN which is greater than all other numbers. This means I don't need to use if (last) {} around the C-style for loop. Feel free to add that if you prefer.

• Use standard formatting with Prettier (avoid smushed together expressions like i+currentHour+1 and strange spacing like {hour: 10, value: 5} ,).
• Avoid using the type in the variable name, so iString should be named what it is only. i generally means "index" so I'd avoid that in the name too.
• Don't use in to iterate arrays because it has gotchas you can read about in the docs. Prefer for ... of.
• Write a test suite to validate the logic thoroughly. This is the sort of function that can have a lot of edge cases. I was bad and didn't test this, so if there's a bug, that only proves my point.

Your input doesn't quite match your code, which is doing some parsing that doesn't appear necessary since you've shown a pre-parsed array/object data structure. I'm going to assume that's the right input. If you need to parse strings, you can do that as a preliminary step. Use JSON.parse() if the input is JSON rather than rolling your own parser.

Good points:

• You're using ===.
• You have a clear function name.
• Your code passes ESLint.

const fillMissingHours = hours => {
  const result = [];
  let last;

  for (const entry of hours) {
    for (let i = last + 1; i < entry.hour; i++) {
      result.push({hour: i, value: 0});
    }

    result.push({...entry});
    last = entry.hour;
  }

  return result;
};

const rawData = [
  {hour: 3, value: 3},
  {hour: 5, value: 9},
  {hour: 10, value: 5},
];
console.log(fillMissingHours(rawData));

I'll admit, this is a wee bit clever because I'm relying on the fact that undefined + 1 is NaN which is greater than all other numbers. This means I don't need to use if (last !== undefined) {} around the C-style for loop. Feel free to add that if you prefer.

• Use standard formatting with Prettier (avoid smushed together expressions like i+currentHour+1 and strange spacing like {hour: 10, value: 5} ,).
• Avoid using the type in the variable name, so iString should be named what it is only. i generally means "index" so I'd avoid that in the name too.
• Don't use in to iterate arrays because it has gotchas you can read about in the docs. Prefer for ... of.
• Write a test suite to validate the logic thoroughly. This is the sort of function that can have a lot of edge cases. I was lazy and didn't test this, so if there's a bug, that only proves my point.

Good points:

• You're using ===.
• You have a clear function name.
• Your code passes ESLint.

Note that your input doesn't quite match your code, which is doing some parsing that doesn't appear necessary since you've shown a pre-parsed array/object data structure. I'm going to assume that's the right input. If you need to parse strings, you can do that as a preliminary step. Use JSON.parse() if the input is JSON rather than rolling your own parser.

As a final item, here's my solution with Array() for comparison. Note that I'm using .push(...[]) to avoid the explicit inner loop.

const fillMissingHours = hours => {
  const result = [];
  let last;

  for (const entry of hours) {
    if (last !== undefined) {
      const fill = [...Array(entry.hour - last - 1)]
        .map((_, i) => ({hour: last + i + 1, value: 0}));
      result.push(...fill);
    }

    result.push({...entry});
    last = entry.hour;
  }

  return result;
};

const rawData = [
  {hour: 3, value: 3},
  {hour: 5, value: 9},
  {hour: 10, value: 5},
];
console.log(fillMissingHours(rawData));

This actually isn't too bad, but you can probably see why I went for simpler syntax.