loggers in expressions of mashup builder

In expressions/validators you can call the console of the Browser: console.log("Hi there")

Don't know what values you want to output in expression but apart from console.log() mentioned by DanZ to log into browser dev tool's console, you can also use below way to output into the mashup logs which can be opened from the top tool bar when viewing Mashups.

•TW.log.info("value is " + val)

•TW.log.debug(xxx)

•TW.log.warning(xxx)

•TW.log.error(xxx)

https://support.ptc.com/help/thingworx/platform/r9/en/index.html#page/ThingWorx/Help/Mashup_Builder/Migration/SupportedRuntimeFunctions.html