Error: uglifyjs not found

I want to minify a folder automatically by a command line. I then installed npm install -g minify-all, then minify-all /opt/myapp/. It gives:

minify-all /opt/myapp/
found file: /opt/myapp/addin_online/css/Common.css
/usr/local/lib/node_modules/minify-all/node_modules/node-minify/lib/node-minify.js:104
          throw new Error(bin + ' not found !');
          ^

Error: uglifyjs not found !
    at getPath (/usr/local/lib/node_modules/minify-all/node_modules/node-minify/lib/node-minify.js:104:17)
    at Object.compress (/usr/local/lib/node_modules/minify-all/node_modules/node-minify/lib/node-minify.js:132:27)
    at Object.minify (/usr/local/lib/node_modules/minify-all/node_modules/node-minify/lib/node-minify.js:83:10)
    at /usr/local/lib/node_modules/minify-all/index.js:30:13
    at /usr/local/lib/node_modules/minify-all/index.js:14:13
    at Array.forEach (native)
    at walk (/usr/local/lib/node_modules/minify-all/index.js:10:36)
    at /usr/local/lib/node_modules/minify-all/index.js:16:13
    at Array.forEach (native)
    at walk (/usr/local/lib/node_modules/minify-all/index.js:10:36)

Does anyone know how to fix this? Is minify-all the right package to use?

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