Gulp 4: gulp.lastRun() incremental builds versus full re-runs for large projects

Answer

gulp.lastRun() does not automatically cover config files outside the gulp.src glob. When you use gulp.src({ since: lastRun(task) }), only files matching the glob pattern and modified since the last successful run are processed. Config files like tsconfig.json or imported templates are invisible to this filter unless explicitly added to the source array or watched separately.

Confirmed facts

• gulp.lastRun(task) returns the timestamp of the last successful task run.
• gulp.src({ since: timestamp })` limits the stream to files modified after that timestamp.
• Files outside the glob are excluded from the since filter, even if they affect compilation output.
• Task failures leave the timestamp unchanged, causing all files to be reprocessed on the next run.
• File deletions are not detected by since; stale outputs persist without a clean step.

Recommended setup

Combine incremental filtering for source changes with a separate full rebuild triggered by config changes:

1. Define the TypeScript compile task with incremental filtering:

const { src, dest, lastRun } = require('gulp');
const ts = require('gulp-typescript');

function compileTs() {
  const tsProject = ts.createProject('tsconfig.json');
  return src(['src/**/*.ts'], { since: lastRun(compileTs) })
    .pipe(tsProject())
    .pipe(dest('dist'));
}


Watch source files for incremental compilation:

function watchSources() {
  watch('src/**/*.ts', compileTs);
}


Create a full rebuild task that cleans and compiles without since:

const del = require('del');

function clean() {
  return del(['dist']);
}

function fullTsBuild() {
  const tsProject = ts.createProject('tsconfig.json');
  return src('src/**/*.ts')
    .pipe(tsProject())
    .pipe(dest('dist'));
}

const build = series(clean, fullTsBuild);


Watch config files and trigger full rebuild on changes:

function watchConfig() {
  watch(['tsconfig.json', 'src/**/*.html', 'src/**/*.template'], build);
}


Export combined watchers:

exports.default = parallel(watchSources, watchConfig);

Missing diagnostic detail
The recommendation assumes no pre-clean step before each incremental compile. If your current workflow already deletes dist before running compileTs, the incremental since filter provides minimal benefit—consider using a simple full rebuild on every change instead for consistency.