Use a library when: 1. It would take a significant time (> a few days) to write yourself. 2. The library is widely used. 3. The library has a history of supp

Use a library when: 1. It would take a significant time (> a few days) to write yourself. 2. The library is widely used. 3. The library has a history of supporting major versions, you don't want breaking changes to come with security and minor fixes. 4. The library is open source. Not always a complete deal breaker, but you need to think of long term costs. 5. You're going to use a significant chunk of the library, don't import a big complicated library for a single function.