Devs-Fix | Quick Solutions for Real-World Code Errors

TypeError: Cannot read properties of undefined (reading 'map') means you called .map() on something that does not exist. JavaScript evaluated the expression to the left of .map , got undefined , and stopped there. The distinction that matters: the array is not empty, it is absent . An empty array maps fine and renders nothing. undefined has no properties at all, so the property lookup itself throws before your callback ever runs. Older JavaScript engines print the same failure as Cannot read property 'map' of undefined . Same bug, older wording - everything on this page applies. Below: the one-line fix, then 11 distinct causes with a reproduction and a fix for each, then a checklist for when your case matches none of them. Error TypeError: Cannot read properties of undefined (reading 'map') Where it happens JavaScript - browser or Node.js, any framework. Most often React or Vue rendering a list from data that has not arrived, or a server response wh...