Variable Scope Solution 2: We can use typeofoperatorto check the variable is defined or not. if (typeof variable !== 'undefined') { // the variable is defined } Solution 3: You can use this code: if (typeof variable === 'undefined') { // variable is undefined } ...
const person = { name: "John" }; if (person.age === undefined) { console.log("Age is not defined"); } 4. Use typeof for Safe Checks Prefer using the typeof operator to check if a variable is undefined, especially when the variable may be undeclared. This method is safe because...
If you want to check whether a variable has been initialized or defined (i.e. test whether a variable has been declared and assigned a value) you can use the typeof operator.The most important reason of using the typeof operator is that it does not throw the ReferenceError if the ...
Now to check whether a given variable is a string or not, we'll use a JavaScript operator calledtypeof. Syntax: typeof variable; This operator returns the data type of the variable specified after it. If the variable is of string data type, it will return a string. Alternatively, it ca...
// Check if variable is equal to value if (username === "sammy_shark") { console.log(true); } 输出: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 true 如前所述,变量可以用来表示任何JavaScript数据类型。在本例中,我们将使用字符串、数字、对象、布尔值和null值声明变量。 代码语言:javascr...
error: Uncaught ReferenceError: somevariable is not defined Note: This isn't to say that typeof is inherently a bad choice - but it does entail this implication as well. Using Lodash to Check if Variable is null, undefined or nil Finally - you may opt to choose external libraries besides...
varx;if(x===undefined){text='x is undefined';}else{text='x is defined';}console.log(text); Output: "x is undefined" Here we take an undefined variable name and compare it directly withundefinedwithout using any function or anything. But this method throws an error if we try to compa...
与其他的语言相比,JavaScript 中 undefined 的概念是有些令人困惑的。特别是试图去理解 ReferenceError(“x is not defined”)以及如何针对它们写出优雅的代码是很令人沮丧的。 本文是我试图把这件事情弄清楚的一些尝试。如果你还不熟悉 JavaScript 中变量和属性的区别(包括内部的 VariableObject),那么最好先去阅读一下...
Method 2: Using the typeof operator to check undefined and null variable: In addition to the OR operator, users can also use the JavaScript typeof operator. It checks the type of variable defined within it. Code Snippet: leta;if(typeofa ==='undefined') {console.log('The variable is un...
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...