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Showing posts with the label scope

Difference between undefined and null

Today we are gonna talk about the famous question in Javascript which all of us must have faced in interview. The difference between undefined and null . But before we talk about their differences, lets first think, why are we comparing undefined and null and why not undefined and string or string and null? So there must be something common,in general, between undefined and null. And that commonality is both of have them represents NO VALUE for the variable. You can also call this NO VALUE as void or empty. So basically when we say as variable is undefined or null that mean the value doesn't exist for the variable. Lets see how is it so. 1.   undefined == null     // true, so the value is same Now lets check each one of them.First lets talk about undefined. When do we get this undefined? Generally when we declare a variable but doesn't initialise it we get undefined. e.g 1.   var foo; 2.   console.log(foo);   // undefined Also when we try to access a vari

Difference between var, let and const

   var, let and const in Javascript        Variable can be declared in three ways: var let const var  variables are function-wide scoped. Let's understand this with an example,    if(true){           var x = 5;    // variable declared with "var"          console.log(x);  // x will be 5    }  console.log(x); // even here, x will be 5          So in the last example we can see variable "x" is available outside the "if" block, that means the scope of variable with "var" is not block scope. let see with another example, 1.  function helloWorld(){       var x = 5;        2.     function abc(){              3.        console.log(x);      // even x will be 5        4.     }        5.     abc();        6.     console.log(x);  // x will be 5 7. } 8. helloWorld();              So in this example, we can see the variable "x" is available throughout the function in which it is declared. One of th