Right now, unknown symbols return undefined, rather than returning a descriptive error:

compileExpression('a', {b: 4})
// undefined
compileExpression('foo of bar')({ bar: 4 })
// undefined

This way, expressions are prone to bugs:

compileExpression('widht > 500', {width: 600})
// false (0)

There should be an API for errors. Errors should be machine-readable and easily localisable by the developer. Errors should be thrown returned much more often.