A brief discussion on null and undefined_javascript techniques in JavaScript

Let’s talk about null first, which represents a special value and is often used to describe “null value”. Perform a typeof operation on null, and the result returns the string "object". In other words, null can be considered as a special object value, which means "non-object" (it feels weird). In fact, null is usually considered to be the only member of its own type, which can represent numbers, strings, and objects as "valueless".

JavaScript also has a second value to represent the vacancy of the value, which is undefined. Undefined value is used to represent a deeper "null value". There are 4 situations when undefined occurs: ① When a variable is declared but not initialized ② When the object property or array element to be queried does not exist ③ If the function does not have any return value, undefined is returned ④ The value of a function parameter that does not provide actual parameters is also referenced will only get undefined.

The two are similar: ① As mentioned earlier, they are both "false values", which means that when JavaScript expects to use a Boolean value, they will be converted to false; ② Neither of them contains any attributes and method.

The difference between the two: ①null is a keyword in the JavaScript language, while undefined is a global variable predefined by JavaScript, not a keyword. Moreover, in ECMAScript 3, undefined is a readable and writable variable, and any value can be assigned to it. This error was corrected in ECMAScript 5. In this version, undefined is read-only (see the current statement on the Internet) Browsers basically support ECMAScript 5. I don’t know why there is no error when I assign a value to undefined in the browser, but its value is not changed); ② When the typeof operation is executed, null returns the "object" string, and undefined returns " undefined" string.

As for comparing null and undefined, null == undefined returns true, null === undefined returns false. You can think of undefined as a vacancy that represents a system-level, unexpected, or error-like value, and null as a vacancy that represents a program-level, normal, or expected value. If you want to assign them to variables or properties or pass them into functions as parameters, it is best to use null.

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