type gurad code example

Example: type gurad

1) Type guards narrow down the type of a variable within a conditional block.
2) Use the 'typeof' and 'instanceof operators to implement type guards in the
	conditional blocks.
   
//example(typeof)
if (typeof a === 'number' && typeof b === 'number') {
    return a + b;
}

(instanceof)
if (job instanceof detail){
	jobless = false ;
}