Function declarations in nested blocks can lead to less readable code and potentially unexpected results due to compatibility issues in different JavaScript runtimes. This does not apply to named or anonymous functions which are valid in a nested block context.

Variables declared with var in nested blocks can also lead to less readable code. Because these variables are hoisted to the module root, it is best to declare them there for clarity. Note that variables declared with let or const are block scoped and therefore this rule does not apply to them.

Invalid:

if (someBool) {
  function doSomething() {}
}

function someFunc(someVal: number): void {
  if (someVal > 4) {
    var a = 10;
  }
}

Valid:

function doSomething() {}
if (someBool) {}

var a = 10;
function someFunc(someVal: number): void {
  var foo = true;
  if (someVal > 4) {
    let b = 10;
    const fn = function doSomethingElse() {};
  }
}