## Usage

Expands numbers and letters, optionally using a `step` as the last argument. _(Numbers may be defined as JavaScript numbers or strings)_.

```js
const fill = require('{%= name %}');
// fill(from, to[, step, options]);

console.log(fill('1', '10')); //=> ['1', '2', '3', '4', '5', '6', '7', '8', '9', '10']
console.log(fill('1', '10', { toRegex: true })); //=> [1-9]|10
```


**Params**

- `from`: **{String|Number}** the number or letter to start with
- `to`: **{String|Number}** the number or letter to end with
- `step`: **{String|Number|Object|Function}** Optionally pass a [step](#optionsstep) to use.
- `options`: **{Object|Function}**: See all available [options](#options)


## Examples

By default, an array of values is returned.

**Alphabetical ranges**

```js
console.log(fill('a', 'e')); //=> ['a', 'b', 'c', 'd', 'e']
console.log(fill('A', 'E')); //=> [ 'A', 'B', 'C', 'D', 'E' ]
```

**Numerical ranges**

Numbers can be defined as actual numbers or strings.

```js
console.log(fill(1, 5));     //=> [ 1, 2, 3, 4, 5 ]
console.log(fill('1', '5')); //=> [ 1, 2, 3, 4, 5 ]
```

**Negative ranges**

Numbers can be defined as actual numbers or strings.

```js
console.log(fill('-5', '-1')); //=> [ '-5', '-4', '-3', '-2', '-1' ]
console.log(fill('-5', '5')); //=> [ '-5', '-4', '-3', '-2', '-1', '0', '1', '2', '3', '4', '5' ]
```

**Steps (increments)**

```js
// numerical ranges with increments
console.log(fill('0', '25', 4)); //=> [ '0', '4', '8', '12', '16', '20', '24' ]
console.log(fill('0', '25', 5)); //=> [ '0', '5', '10', '15', '20', '25' ]
console.log(fill('0', '25', 6)); //=> [ '0', '6', '12', '18', '24' ]

// alphabetical ranges with increments
console.log(fill('a', 'z', 4)); //=> [ 'a', 'e', 'i', 'm', 'q', 'u', 'y' ]
console.log(fill('a', 'z', 5)); //=> [ 'a', 'f', 'k', 'p', 'u', 'z' ]
console.log(fill('a', 'z', 6)); //=> [ 'a', 'g', 'm', 's', 'y' ]
```

## Options

### options.step

**Type**: `number` (formatted as a string or number)

**Default**: `undefined`

**Description**: The increment to use for the range. Can be used with letters or numbers.

**Example(s)**

```js
// numbers
console.log(fill('1', '10', 2)); //=> [ '1', '3', '5', '7', '9' ]
console.log(fill('1', '10', 3)); //=> [ '1', '4', '7', '10' ]
console.log(fill('1', '10', 4)); //=> [ '1', '5', '9' ]

// letters
console.log(fill('a', 'z', 5)); //=> [ 'a', 'f', 'k', 'p', 'u', 'z' ]
console.log(fill('a', 'z', 7)); //=> [ 'a', 'h', 'o', 'v' ]
console.log(fill('a', 'z', 9)); //=> [ 'a', 'j', 's' ]
```

### options.strictRanges

**Type**: `boolean`

**Default**: `false`

**Description**: By default, `null` is returned when an invalid range is passed. Enable this option to throw a `RangeError` on invalid ranges.

**Example(s)**

The following are all invalid:

```js
fill('1.1', '2');   // decimals not supported in ranges
fill('a', '2');     // incompatible range values
fill(1, 10, 'foo'); // invalid "step" argument
```

### options.stringify

**Type**: `boolean`

**Default**: `undefined`

**Description**: Cast all returned values to strings. By default, integers are returned as numbers.

**Example(s)**

```js
console.log(fill(1, 5));                    //=> [ 1, 2, 3, 4, 5 ]
console.log(fill(1, 5, { stringify: true })); //=> [ '1', '2', '3', '4', '5' ]
```

### options.toRegex

**Type**: `boolean`

**Default**: `undefined`

**Description**: Create a regex-compatible source string, instead of expanding values to an array.

**Example(s)**

```js
// alphabetical range
console.log(fill('a', 'e', { toRegex: true })); //=> '[a-e]'
// alphabetical with step
console.log(fill('a', 'z', 3, { toRegex: true })); //=> 'a|d|g|j|m|p|s|v|y'
// numerical range
console.log(fill('1', '100', { toRegex: true })); //=> '[1-9]|[1-9][0-9]|100'
// numerical range with zero padding
console.log(fill('000001', '100000', { toRegex: true }));
//=> '0{5}[1-9]|0{4}[1-9][0-9]|0{3}[1-9][0-9]{2}|0{2}[1-9][0-9]{3}|0[1-9][0-9]{4}|100000'
```

### options.transform

**Type**: `function`

**Default**: `undefined`

**Description**: Customize each value in the returned array (or [string](#optionstoRegex)). _(you can also pass this function as the last argument to `fill()`)_.

**Example(s)**

```js
// add zero padding
console.log(fill(1, 5, value => String(value).padStart(4, '0')));
//=> ['0001', '0002', '0003', '0004', '0005']
```