At work we often need to determine whether a variable/property is undefined. There are usually two ways to write it

//Method 1
typeof age === 'undefined';

//Way 2
age === undefined

What’s the difference? Which one should I use? Take a look at the following example

typeof age === 'undefined'; // true

The identifier age is not declared, and the output is true.

Let’s do another example

age === undefined; //  An error

Firebug says age is not defined,

This is the difference between the two, that is, if you are not sure whether age is declared or defined, you can use mode 1; if you are sure, you can use mode 2. Use mode 1: if the variable is not declared, the code will not report an error, but mode 2 will report an error. It seems that way 1 is more fault-tolerant, but actually it will be a latent Bug. It is always a good habit to declare variables before using them.

In addition, mode 1 is two operations and mode 2 is one.