How to transfer music from my iPhone to iTunes on my laptop?

Ever since i got my iphone i have been buying music on my iphone and then putting it on itunes on my computer but a few months ago i got a new computer and i signed into my itunes account and i thought all my music would automatically download but it didn't. When i go on music and click "sync" it says "all music and playlists on iphone will be replaced with music and playlists on itunes" but since i have no music on itunes i'm worried that i'll loose all my music. How can i transfer music from my iphone onto itunes without taking it off my phone?

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You may take a look at this step by step guide, it's about how to transfer files from iPod touch and iPhone to computer iTunes, it supports Apps, Contacts, Songs, Videos, Photos, Playlist, Games, etc. Data files. Easy to understand and works pretty well for my iPod touch.

Get "Diskaid version 5" It iIncludes TuneAid, which it can copy the music frOm Iphone to your PC. No need for jailbreak.