Hi,
Yes, that is if you use static functions (in this case IF is a static data function)
Of course, your expression could’ve been like:
“content.substring(0,150)” - if you type the expression like that, then the field would always return a string of length 150 characters.
With what we set-up above, now you return a substring if its larger then certain value, else the full one.
Depending on your use case - for example if i understood correctly, you want to show something like “Some content… See more…” that expands to the full text. So i can imagine your front-end developer could also make use of the following setup for your data definition - you could have 2 fields:
-short_content - string -content.substring(0,150)
-full_content - string - content
Maybe even a boolean field:
-has_really_long_content - content.length().isgreater(150) (the isgreater
evaluates to a boolean, as you saw above)
Now in the same data definition result you have both results which your front-end developer can utilise further…
But i leave the further setup to your imagination 
To find out more about expressions you can also refer to this Wiki:
It explains the above in more detail.