While both retinol and Retin-A contain retinoids, they differ in their concentration. Retinol and other forms, such as retinal and retinyl palmitate are found in many over-the-counter anti-aging creams. While these all contain a form of vitamin A, the concentration of retinoids is weak. Retin-A is a prescription retinoid and is a brand name for tretinoin, which is the carboxylic acid form of vitamin A and was one of the early topical acne treatments. Retin-A is sold by prescription because its acid composition is much greater than in over-the-counter products.
Undefined is a term used when a mathematical result has no meaning. More precisely, undefined values occur when an expression is evaluated for input values outside of its domain. ( 1 if no complex numbers). ( 2 if no complex numbers). ( 3 units in radians, no complex infinity). ( 4 if no complex infinity).
The only way to truly test if a variable is undefined is to do the following. Remember, undefined is an object in Javascript. To determine if a variable or object property exists, It is good to compare to keyword undefined or you can also check the type of the object to string âundefinedâ. In the following JavaScript code we compare the num to the undefined in both way.