The Number.isFinite() function checks if the variable is a number, but also checks if it's a finite value. Therefore, it returns false on numbers that are NaN, Infinity or -Infinity. Let's test it out on the variables we've defined above: > Number.isFinite(intVar); true > Number...
Methods to Check if a Variable is Undefined Using typeof Operator Using Strict Equality (===) Using Loose Equality (==) Using the void Operator Using a Custom Utility Function This article covers different ways to check if a variable is undefined, helping you write code that handles these ...
}// ✅ Check if nullif(message ===null) {console.log('variable is null'); }// ✅ 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 un...
JavaScript check if variable exists (is definedinitialized) - We can use typeof operator to check the variable is defined or not.
let myString = "John Doe"; if (typeof myString === "string") { console.log("This variable is a string"); } else { console.log("This variable is not a string"); } Indeed, the myString is a string: This variable is a string Note: Even if the variable contains a number th...
This example shows how to check whether the value of an input field is a valid number. We setonChangethe property on the input field, so every time its value changes,handleChangethe function will be called. In ourhandleChangefunction, we updatenumthe state variable and check if the value ...
In those applications, as one example, if a variable arrives at zero and it loses its sign, then you would lose the information of what direction it was moving in before it arrived at zero. Preserving the sign of the zero prevents potentially unwanted information loss....
variable instanceof Array This method runs about 1/3 the speed as the first example. Still pretty solid, looks cleaner, if you're all about pretty code and not so much on performance. Note that checking for numbers does not work asvariable instanceof Numberalways returnsfalse.Update:instance...
variable instanceof Array 1. This method runs about1/3 the speedas the first example. Still pretty solid, looks cleaner, if you're all about pretty code and not so much on performance. Note that checking for numbers does not work as variable instanceof Number always returns...
{// let and constVariableDeclaration(node) {if (node.kind === 'let' || node.kind === 'const') {errArr.push({node,message: `Using ${node.kind} is not allowed`})}},// 箭头函数ArrowFunctionExpression(node) {errArr.push({node,message: 'Using ArrowFunction(箭头函数) is not allowed...