const arr2 = []; 
// const arr2: any[]
arr2[0] = '1';
arr2; 
// const arr2: string[]
arr2[1] = 3;
arr2; 
//  const arr2: (string | number)[]

const arr3 = []; 
// Variable 'arr3' implicitly has type 'any[]' in some locations where its type cannot be determined.
const arr4 = arr3.concat('123'); 
// Variable 'arr3' implicitly has an 'any[]' type.