React, using .map with index

You need to to this:

todos.map((key, index) => { ... }) without object brackets for arguments.

({todo, index}) => { ... } – that syntax means that you want to get properties todo and index from first argument of function.

You can see syntax here:

Basic syntax

 (param1, param2, …, paramN) => { statements }
 (param1, param2, …, paramN) => expression
          // equivalent to:  => { return expression; }

 // Parentheses are optional when there's only one parameter:
 (singleParam) => { statements }
 singleParam => { statements }

 // A function with no parameters requires parentheses:
 () => { statements }

Advanced syntax

 // Parenthesize the body to return an object literal expression:
 params => ({foo: bar})

 // Rest parameters and default parameters are supported
 (param1, param2, ...rest) => { statements }
 (param1 = defaultValue1, param2, …, paramN = defaultValueN) => { statements }

 // Destructuring within the parameter list is also supported
 var f = ([a, b] = [1, 2], {x: c} = {x: a + b}) => a + b + c;
 f();  // 6