問題

% ./node_modules/.bin/webpack
# 中略
TypeError: Cannot read property 'watchRun' of undefined

調査

• webpack - TypeScript issue - TypeError: Cannot read property 'watchRun' of undefined - Stack Overflow
• webpack 4.0.0-alpha.5 feedback · Issue #6357 · webpack/webpack · GitHub

解決

% ./node_modules/.bin/webpack --version
3.11.0
% npm install --save-dev -D webpack@4.0
% ./node_modules/.bin/webpack --version
4.0.1

% ./node_modules/.bin/webpack
# うごいた