Variable equals another variable and then equals another variable

In JavaScript (and PHP, too), you can save a few lines of code by assigning multiple variables at once. In this example, the function wp.media() is called and immediately assigned to wp.media.frames.file_frame. The author wanted to use the same value for mediaUploader so they added it in the same assignment statement.

Another way to look at it is

// Option 1: single function call, multiple assignments    
wp.media.frames.file_frame = wp.media({...});
mediaUploader              = wp.media.frames.file_frame;

// Option 2: multiple function calls, multiple assignments
wp.media.frames.file_frame = wp.media({...});
mediaUploader              = wp.media({...});

This can be useful if you’re assigning a value to a global object but want to manipulate the value locally without affecting the global. In this case, the author can assign the value to wp.media.frames.file_frame so it’s available there, and use mediaUploader locally inside a function without affecting the value of the other scoped variable.