Undefined is a type of Data type in JavaScript. When a variable is declared and not initialized or not assigned with any value. By default, the variable was stored with an Undefined value.
Undefined is a global read-only variable that represents the value Undefined. Undefined is a type of primitive type
Syntax:
let variable = undefined; // or let no;
Both no and variable contain an undefined value.
1. Undefined Value in variables: When a variable is declared or not initialized but not assigned with any value then JavaScript automatically assigns it to an “undefined” value.
Example: In the below example, the variable name is declared but not assigned with any value, so it gets an Undefined value:
Javascript
let newVar;console.log(newVar) |
Output:
undefined
2. Undefined Value using Functions: A method or statement also returns undefined. If a variable was assigned with a function that does not return any value, then the JavaScript assigns an undefined value to that variable.
Example: In the below example sayhi() function actually outputs and returns nothing. In the above, we assigned the sayhi function to the x variable, so we get an Undefined value in the x variable as the sayhi() function returns nothing.
Javascript
function sayhi(name) { console.log(`hi ${name}`);}x = sayhi('hike');console.log(`value in x= ${x}`); |
Output:
hi hike value in x=undefined
