@stdlib/string/starts-with/README.md

String manipulation functions.

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# startsWith

> Test if a string starts with the characters of another string.

<section class="intro">

</section>

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<section class="usage">

## Usage

```javascript
var startsWith = require( '@stdlib/string/starts-with' );
```

#### startsWith( str, search\[, position] )

Tests if a string starts with the characters of another string.

```javascript
var str = 'To be, or not to be, that is the question.';

var bool = startsWith( str, 'To be' );
// returns true

bool = startsWith( str, 'to be' );
// returns false
```

By default, the function searches from the beginning of the input string. To search from a different character index, provide a `position` value (zero-based). If provided a negative `position`, the start index is determined relative to the string end (i.e., `pos = str.length + position`).

```javascript
var str = 'Remember the story I used to tell you when you were a boy?';

var bool = startsWith( str, 'the story' );
// returns false

bool = startsWith( str, 'the story', 9 );
// returns true

bool = startsWith( str, 'you', -15 );
// returns true
```

If provided an empty `search` string, the function **always** returns `true`.

```javascript
var str = 'beep boop';

var bool = startsWith( str, '' );
// returns true
```

</section>

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<section class="notes">

## Notes

-   This function differs from [`String.prototype.startsWith`][mdn-string-startswith] in the following ways:

    -   The function requires string values for the first and second arguments and requires that the `position` argument be an integer value.
    -   The function does **not** clamp negative `position` values. Instead, when provided a negative `position`, the function resolves the starting search position relative to the end of the string.
    -   Except when provided an empty `search` string, the function **always** returns `false` if a `position` resolves to a starting search position which exceeds the bounds of the input string.

</section>

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<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var startsWith = require( '@stdlib/string/starts-with' );

var str = 'Fair is foul, and foul is fair, hover through fog and filthy air';

var bool = startsWith( str, 'Fair' );
// returns true

bool = startsWith( str, 'fair' );
// returns false

bool = startsWith( str, 'foul', 8 );
// returns true

bool = startsWith( str, 'filthy', -10 );
// returns true
```

</section>

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* * *

<section class="cli">

## CLI

<section class="usage">

### Usage

```text
Usage: starts-with [options] --search=<string> [<string>]

Options:

  -h,    --help                Print this message.
  -V,    --version             Print the package version.
         --search string       Search string.
         --pos int             Search position.
         --split sep           Delimiter for stdin data. Default: '/\\r?\\n/'.
```

</section>

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<!-- CLI usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

### Notes

-   If the split separator is a [regular expression][mdn-regexp], ensure that the `split` option is either properly escaped or enclosed in quotes.

    ```bash
    # Not escaped...
    $ echo -n $'Hello, World!\nBeep Boop Baz' | starts-with --search=Beep --split /\r?\n/

    # Escaped...
    $ echo -n $'Hello, World!\nBeep Boop Baz' | starts-with --search=Beep --split /\\r?\\n/
    ```

-   The implementation ignores trailing delimiters.

</section>

</section>

<!-- /.notes -->

<section class="examples">

### Examples

```bash
$ starts-with --search=be beep
true
```

To use as a [standard stream][standard-streams],

```bash
$ echo -n 'boop' | starts-with --search=bo
true
```

By default, when used as a [standard stream][standard-streams], the implementation assumes newline-delimited data. To specify an alternative delimiter, set the `split` option.

```bash
$ echo -n 'Hello, World!\tBeep Boop' | starts-with --search=Beep --split '\t'
false
true
```

</section>

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</section>

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<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/string/ends-with`][@stdlib/string/ends-with]</span><span class="delimiter">: </span><span class="description">test if a string ends with the characters of another string.</span>

</section>

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<section class="links">

[standard-streams]: https://en.wikipedia.org/wiki/Standard_streams

[mdn-regexp]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions

[mdn-string-startswith]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith

<!-- <related-links> -->

[@stdlib/string/ends-with]: https://github.com/stdlib-js/string/tree/main/ends-with

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</section>

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