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).
Understanding JavaScriptâs âundefinedâ. Compared to other languages, JavaScriptâs concept of undefined is a little confusing. In particular, trying to understand ReferenceErrors (âx is not definedâ) and how best to code against them can be frustrating.This is my attempt to straighten things out a little.n JavaScript there is Undefined (type), undefined (value) and undefined (variable). Undefined (type) is a built-in JavaScript type. undefined (value) is a primitive and is the sole value of the Undefined type. Any property that has not been assigned a value, assumes the undefined value. (ECMA 4.3.9 and 4.3.10).