Knowledge about JavaScript typeof, null, and undefined
JavaScript typeof, null, and undefined operators play a very important role in js. Let us explain them in detail.
typeof operator
You can use the typeof operator to detect the data type of a variable.
Example
typeof "John" // 返回 string
typeof 3.14 // 返回 number
typeof false // 返回 boolean
typeof [1,2,3,4] // 返回 object
typeof {name:'John', age:34} // 返回 objectIn JavaScript, an array is a special object type. So typeof [1,2,3,4] returns object.
null
In JavaScript, null means "nothing".
null is a special type with only one value. Represents an empty object reference.
#Use typeof to detect null and return object.
You can set it to null to clear the object:
Instance
var person = null; // 值为 null(空), 但类型为对象
You can set it to undefined to clear the object:
Instance
var person = undefined; // 值为 undefined, 类型为 undefined
undefined
In JavaScript, undefined is a variable that has no set value.
typeof A variable with no value will return undefined.
Example
var person; // 值为 undefined(空), 类型是undefined
Any variable can be cleared by setting the value to undefined. The type is undefined.
Instance
person = undefined; // 值为 undefined, 类型是undefined
The difference between undefined and null
Instance
The values of null and undefined are equal, but the types are not the same:
typeof undefined // undefined typeof null // object null === undefined // false null == undefined // true
This article explains in detail the knowledge related to JavaScript typeof, null, and undefined. For more learning materials, please pay attention to the php Chinese website.
Related recommendations:
JavaScript Date (date) related knowledge and usage
Introduction to the use of JavaScript RegExp objects
About how to use JavaScript Array (array) object
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