Category Archives: Interview Questions

Interview questions

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  1. What is difference between let and var while declaring variable?
    var is Function Scoped and let is block scoped
    Using var

    var foo = 123;
if (true) {
    var foo = 456;
}

console.log(foo); // 456

    Using let

    let foo = 123;
if (true) {
    let foo = 456;

}
console.log(foo); // 123
  2. What would be the value of the below variables? 
     
 //Value is Undefined
 var foo:string;
 
 //You can explicitly define value as undefined 
 var age = undefined;

 //any is a valid value which is default type until the variable assumes some data type by type inference
 var location:any;

 //undefined and null could be assigned to any of the datatype
 var pincode:any=undefined;
 var state:any=null;
  3. Why the below is not possible in interface?
    I am having a interface and class like one below which results in compile time exception

    interface test1 {
}

class test2 implements test1
{
    public foo;
}

let test: test1 = new test2();

test.foo = 'test';

    The reason for the compilation error is when you reference a variable with specific interface in TypeScript, you can only use the properties and methods declared in the interface of the variable
    You are assigning test1 as a type of test variable, test1 interface doesn’t have foo property in it. so that’s why you are getting this error. If you change the type to let test: test2: new test2();. it won’t throw any error like one below

    let test: test2 = new test2();
test.foo = 'test';

    There are two workaround for this. One is not to define the type while initializing variable like one below

    interface test1 { }
class test2 implements test1{
    public foo;
}
let test = new test2();
test.foo = 'test';

    Other is to use any while initializing variable

    interface test1 { }
class test2 implements test1{
    public foo;
}
let test: test1 | any = new test2();
test.foo = 'test';