Pro Intelligent code completion
WebStorm has two types of autocompletion: structural completion and word expansion.
Both types of autocompletion work extremely well, have little to no problems and are quite fast even when loading suggestions on the go.
Pro Code refactoring support
To make sure your code can be easily maintained, you must first be sure to make it clean and tidy. This is the part where WebStorm really helps you. It automatically refactors your code by performing functions on it, such as extraction of variables, moving files, inline variable extraction, etc.
Pro Integrated code quality tools
WebStorm comes bundled with JSHint and JSLint. JSCS, ESLint, and Closure Linter can be installed via npm. They register as inspections and are customizable through IDE settings. They run automatically and will highlight potential issues. Pressing alt+enter on an issue will allow the user to view suggested fixes.
Pro Built-in terminal/run tools
WebStorm (and really all of the IntelliJ IDEs) support the plugins throughout their plugin ecosystem which leaves you with 100s of tools to handle your automation tasks. There is a wide range of build-related plugins that help you by having pre-defined commands to execute with the click of a button. Out of any other IDE, WebStorm has by far the most coverage when it comes to tools for your development workflow.