So that you can use typescript along all projects with same version and do tsc --watch to build js from it.
Actually for majority it is not a prefer way as different developer may be using different version of typescript. So you can also do npm i --save-dev typescript or install extension from code editor.
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First, you need to install typescript and tslint globally with -g option before installing them in devDependencies in your project.
why?
So that you can use typescript along all projects with same version and do tsc --watch to build js from it.
Actually for majority it is not a prefer way as different developer may be using different version of typescript. So you can also do npm i --save-dev typescript or install extension from code editor.