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extname.ts
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extname.ts
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// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.
import { CHAR_DOT } from "../_common/constants.ts";
import { assertPath } from "../_common/assert_path.ts";
import { isPosixPathSeparator } from "./_util.ts";
/**
* Return the extension of the `path` with leading period.
*
* @example
* ```ts
* import { extname } from "https://deno.land/std@$STD_VERSION/path/extname.ts";
*
* console.log(extname("/home/user/Documents/")); // ""
* console.log(extname("/home/user/Documents/image.png")); // ".png"
* ```
*
* @param path with extension
* @returns extension (ex. for `file.ts` returns `.ts`)
*/
export function extname(path: string): string {
assertPath(path);
let startDot = -1;
let startPart = 0;
let end = -1;
let matchedSlash = true;
// Track the state of characters (if any) we see before our first dot and
// after any path separator we find
let preDotState = 0;
for (let i = path.length - 1; i >= 0; --i) {
const code = path.charCodeAt(i);
if (isPosixPathSeparator(code)) {
// If we reached a path separator that was not part of a set of path
// separators at the end of the string, stop now
if (!matchedSlash) {
startPart = i + 1;
break;
}
continue;
}
if (end === -1) {
// We saw the first non-path separator, mark this as the end of our
// extension
matchedSlash = false;
end = i + 1;
}
if (code === CHAR_DOT) {
// If this is our first dot, mark it as the start of our extension
if (startDot === -1) startDot = i;
else if (preDotState !== 1) preDotState = 1;
} else if (startDot !== -1) {
// We saw a non-dot and non-path separator before our dot, so we should
// have a good chance at having a non-empty extension
preDotState = -1;
}
}
if (
startDot === -1 ||
end === -1 ||
// We saw a non-dot character immediately before the dot
preDotState === 0 ||
// The (right-most) trimmed path component is exactly '..'
(preDotState === 1 && startDot === end - 1 && startDot === startPart + 1)
) {
return "";
}
return path.slice(startDot, end);
}