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typeof Output “undefined” “object” (historical quirk) Common Usage Variables, function return values, missing object properties To represent a variable that is intentionally empty Example let a; console.log(a); // undefined let b = null; console.log(b); // null undefined and null are ...
alert(typeof undefined); //output "undefined" alert(typeof null); //output "object" 第一行代码很容易理解,undefined的类型为Undefined;第二行代码却让人疑惑,为什么null的类型又是Object了呢?其实这是JavaScript最初实现的一个错误,后来被ECMAScript沿用下来。在今天我们可以解释为,null即是一个不存在的对象...
When accessing an object property or array element that doesn't exist, it results in undefined.var x; console.log(x); // Output: undefined function exampleFunc(y) { console.log(y); // Output: undefined } exampleFunc(); var obj = { key: "value" }; console.log(obj.nonExistent); ...
Learn how to count undefined elements in a JavaScript array with simple examples and step-by-step explanations.
Arrays in JavaScript can be sparse, which introduces yet another way to create missing values. arr1=[]arr1[0]="Zero"arr1[2]="Two" If I emitarr1to the console, I getArray(3) [ “Zero”, <1 empty slot>, “Two” ]as the short output, at least in Firefox. Except, if I expand...
The codein front ofthe outputof the twovariablesare"undefined",even if onlyvariableoTemp2is undefined.If youdo nottypeofoperatortouse otheroperatorson theoTemp2, which willcause an error,because theoperatoronly forthe definedvariables.For example, the following codewill throwan error: ...
Understand the key differences between undefined and null in JavaScript. Learn how each is used and when to apply them effectively.
为什么JavaScript同时有null和undefined？我建议你想想他们的目的，以便更好地理解两者的区别。值null表示...
要区分它们,首先要明确这两个值所代表的意义: null 是一个 JavaScript字面量,表示空值,即没有对象被呈现,它是JavaScript的原始值之一,使用typeof运算得到 “object”。...console.log(x ===y); //false 在上面的代码中 == 会继续类型转换,且值是相等的,所...