JavaScript | Check the existence of variable
JavaScript has a bulit-in function to check whether a variable is defined/initialized or undefined.
Note:
- The typeof operator will check whether a variable is defined or not.
- The typeof operator doesn’t throw a ReferenceError exception when it is used with an undeclared variable.
- The typeof null will return an object. So, check for null also.
Example 1: This example checks whether a variable is defined or not.
<!DOCTYPE html> < html > < head > < title > JavaScript to check existence of variable </ title > </ head > < body style = "text-align:center;" > < h1 style = "color:green;" > GeeksForGeeks </ h1 > < p > variable-name : GFG_Var </ p > < button onclick = "myGeeks()" > Check </ button > < h3 id = "div" style = "color:green;" ></ h3 > <!-- Script to check existence of variable --> < script > function myGeeks() { var GFG_Var; var h3 = document.getElementById("div"); if (typeof GFG_Var === 'undefined') { h3.innerHTML = "Variable is Undefined"; } else { h3.innerHTML = "Variable is defined and" + " value is " + GFG_Var; } } </ script > </ body > </ html > |
Output:
- Before clicking the button:
- After clicking the button:
Example 2: This example also checks whether a variable is defined or not.
<!DOCTYPE html> < html > < head > < title > JavaScript to check existence of variable </ title > </ head > < body style = "text-align:center;" > < h1 style = "color:green;" > GeeksForGeeks </ h1 > < p > variable-name : GFG_Var </ p > < button onclick = "myGeeks()" > Check </ button > < h3 id = "div" style = "color:green;" ></ h3 > <!-- Script to check existence of variable --> < script > function myGeeks() { var GFG_Var = "GFG"; var h3 = document.getElementById("div"); if (typeof GFG_Var === 'undefined') { h3.innerHTML = "Variable is Undefined"; } else { h3.innerHTML = "Variable is defined and" + " value is " + GFG_Var; } } </ script > </ body > </ html > |
Output:
- Before clicking the button:
- After clicking the button:
Example 3: The previous example doesn’t check for null value of variable. This example also checks a variable is null or not.
<!DOCTYPE html> < html > < head > < title > JavaScript to check existence of variable </ title > </ head > < body style = "text-align:center;" > < h1 style = "color:green;" > GeeksForGeeks </ h1 > < p > variable-name : GFG_Var </ p > < button onclick = "myGeeks()" > Check </ button > < h3 id = "div" style = "color:green;" ></ h3 > <!-- Script to check existence of variable --> < script > function myGeeks() { var GFG_Var = null; var h3 = document.getElementById("div"); if (typeof GFG_Var === 'undefined' || GFG_Var === null ) { h3.innerHTML = "Variable is Undefined"; } else { h3.innerHTML = "Variable is defined and value is " + GFG_Var; } } </ script > </ body > </ html > |
Output:
- Before clicking the button:
- After clicking the button: