“this” is undefined inside map function Reactjs

Array.prototype.map() takes a second argument to set what this refers to in the mapping function, so pass this as the second argument to preserve the current context:

someList.map(function(item) {
  ...
}, this)

Alternatively, you can use an ES6 arrow function to automatically preserve the current this context:

someList.map((item) => {
  ...
})