typescript code snippets 02

const arr: number[] = [1, 2, 3];
arr.push(4);

const myObj: { x: number } = { x: 1 };
myObj.x = 2;

let n2: string | undefined = Math.random() > 0.5 ? undefined : "test";
// console.log(n2.substring(0, 1)); // Won't compile since can be null

if (n2 !== null) {
  console.log(n2.substring(0, 1));
}

function f1(optional?: string): void {
  if (optional === undefined) {
    // Optional parameter was not provided OR was set to undefined
  } else {
    // The optional parameter is for sure a string (not undefined)
  }
}

let primitiveWithUndefined: number | undefined = undefined;

function functOptionalArg(primitiveOptional?: number): void {
  // ...
}
functOptionalArg();
functOptionalArg(undefined);
functOptionalArg(1);