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difference between .jsm and .js files

i had researched properly for the .jsm and .js files . and i get that both are JavaScript files but want to know where it should be use .jsm file and where to use .js file . is there any plus or minus between them or both are same , or no difference .
so if something good about .jsm file , or any extra features , i want to use it in my next web application

so whats difference between .jsm file and .js file ?

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