Variable definitions

Classical var is working to be compatible with javascript

var simpleVar = 1;

It has problems with scoping, so it is usually forbidden to use by tsc options

Recommended to use:

• let for variables changing values
• const for constants having one value all time.

const databaseAdapter = {};
let index = 0;

You could still modify internal properties of objects and arrays defined with const:

const object = {
  a: 1,
  b: 2,
};
object.a = 10;

object = {a: 10, b:2 } // will not work

const myArray = [];
myArray.push(10); // ok

Object key short notation

const userId = 10;
const userObject = {
  userId, // this mean: take `userId` as property name and assign value from `userId` variable
};

Block-scoping

function checkForError(isError: boolean) {
  let errorPrefix = "Error: ";
  if (isError) {
    let fullError = errorPrefix + "here";
    return fullError;
  }
  // @ts-ignore
  return fullError; // Error: 'fullError' doesn't exist here
}

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let works within scope and could be shadowed within child scope

function checkForErrorAnother(isError: boolean) {
  const errorPrefix = "Error: ";
  if (isError) {
    const errorPrefix = "Internal Error: ";
    let fullError = errorPrefix + "here";
    return fullError;
  }
  return errorPrefix + " not happened";
}
const result1 = checkForErrorAnother(true); // "Internal Error: here"
const result2 = checkForErrorAnother(false); // "Error:  not happened"
console.log(result1);
console.log(result2);

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