In JavaScript, undefined is the default value for variables that have been declared but not initialized. On the other hand, null is an intentional assignment that explicitly indicates the absence of a value. Methods to Check if a Variable is Undefined Using typeof Operator Using Strict Equality...
Thetypeofoperator can additionally be used alongside the===operator to check if the type of a variable is equal to'undefined'or'null': leta;if(typeofa ==='undefined') {console.log('Undefined variable'); }elseif(typeofa ==='null') {console.log('Null-value'); } ...
So, we can check that if the variable contains the null value or undefined value, it means we have a null variable.SyntaxBelow is the syntax given for checking the undefined value of the variable using the strict equality operator.variable_name === undefined ...
JavaScript can make it difficult to distinguish between different undefined values. Boolean operations also have a couple of gotchas you must be aware of when dealing withNaNvalues. There are situations where one has to check and see if a certain value is of typeNaN. As such, here are some...
let myVar; if (typeof myVar === "undefined") { console.log("myVar is undefined"); } else { console.log("myVar is defined"); } Output: myVar is undefined In this example, we declare a variable myVar without assigning any value to it. When we check its type using typeof, we ...
Check if the input value is a valid number in React: Check if the input value is not an empty string or contains only spaces. Pass the value to the isNaN() function. If isNaN returns false , then the value is a valid number. import {useState} from rea
Ensure the value is notundefinedornullusingArray.prototype.includes(). Compare theconstructorproperty on the value withtypeto check if the provided value is of the specifiedtype. constis=(type,val)=>![,null].includes(val)&&val.constructor===type;// Examplesis(Array,[1]);// trueis(ArrayBuff...
leta;if(typeofa ==='undefined') {console.log('The variable is undefined '); }elseif(typeofa ==='null') {console.log('The variable is Null-value'); } Output: Run Code Explanation: The typeof operator specifies the type of the variable. However, users should note that if they chuc...
--文本框的备选项--><datalist v-if="typeof(meta.optionKey)!=='undefined'":id="meta.optionKey"></datalist></template> 代码语言:javascript 复制 exportdefault{name:'nf-form-input',model:{prop:'modelValue',event:'input'},props:{
}// ✅ Check if value is NOT equal to null and undefinedif(message !==undefined&& message !==null) {console.log('variable is NOT undefined or null'); }// ✅ Check if value is equal to null or undefinedif(message ===undefined|| message ===null) {console.log('variable is undef...