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* @file minify.js
* @description Removes what a browser does not need, and nothing else.
*
* ## What this does, stated precisely
*
* Comments are deleted. Leading and trailing whitespace is trimmed from every
* line whose margins are code rather than the inside of a string or a template
* literal. That is the whole transformation.
*
* It does **not** rename identifiers, fold constants, remove dead branches or
* join statements. Those need a real ECMAScript parser to do safely, and a
* minifier that guesses produces a bundle that is smaller and wrong — which is
* strictly worse than one that is larger and right. The size this leaves on the
* table is reported honestly rather than closed by guessing.
*
* ## One property worth the constraint
*
* Line count is preserved exactly. A deleted block comment leaves its newlines
* behind; trimming touches only the margins. So the source map emitted for a
* development build is equally valid for the minified production build, and a
* production stack trace still names a file and a line the developer wrote.
* Most minifiers buy their last few per cent by giving that up.
*
* Comments and indentation are also what gzip compresses best, so the gap
* between this and an identifier-mangling minifier is much narrower on the wire
* than it is on disk. The build reports both numbers.
* @module lib/bundler/minify
*/
import { CodeMask } from './scanner.js';
/**
* Strips comments and indentation from emitted JavaScript.
* @param {string} code - The bundle source.
* @returns {string} The same program, with comments and margins removed.
*/
export function minify(code) {
const mask = new CodeMask(code);
// Comment spans, so a line that is entirely comment collapses to nothing and
// one that ends in a comment loses the tail. Recorded as ranges rather than
// by rewriting a character array: the bundle is hundreds of kilobytes and
// this runs on every build, including every rebuild `avenx watch` performs.
const comment = new Uint8Array(code.length);
for (const region of mask.regions) {
if (region.kind !== 'line-comment' && region.kind !== 'block-comment') continue;
for (let i = region.start; i < Math.min(region.end, code.length); i += 1) {
comment[i] = 1;
}
}
/**
* Whether an offset can be dropped from a line's margin.
*
* A blanked comment can; a string or template literal's own spacing cannot,
* because it is a value. That distinction is the whole safety argument for
* this minifier, and getting it backwards would corrupt every multi-line
* template in the runtime.
* @param {number} index - Offset to test.
* @returns {boolean} True when the character is droppable margin.
*/
const droppable = (index) => {
if (comment[index] === 1) return true;
const char = code[index];
return (char === ' ' || char === '\t' || char === '\r') && !mask.isLiteral(index);
};
const out = [];
let lineStart = 0;
/**
* Emits one line, trimmed where trimming is safe.
* @param {number} end - Exclusive end offset of the line.
*/
const flush = (end) => {
let from = lineStart;
let to = end;
while (from < to && droppable(from)) from += 1;
while (to > from && droppable(to - 1)) to -= 1;
if (to <= from) {
out.push('');
return;
}
// A comment in the middle of a line -- `const x = 1; // why` is handled by
// the trailing trim, but `foo(/* note */ bar)` is not -- so the interior is
// copied with comment characters replaced by a single space each.
let piece = '';
let spanStart = from;
for (let i = from; i < to; i += 1) {
if (comment[i] !== 1) continue;
piece += code.slice(spanStart, i);
while (i < to && comment[i] === 1) i += 1;
piece += ' ';
spanStart = i;
}
out.push(spanStart === from ? code.slice(from, to) : piece + code.slice(spanStart, to));
};
for (let i = 0; i < code.length; i += 1) {
if (code[i] === '\n') {
flush(i);
lineStart = i + 1;
}
}
flush(code.length);
return out.join('\n');
}
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