在 Javascript 中检查 undefined'ness 时，我应该使用 != 还是 !==？

Question

When I want to make sure a variable x is defined, before using it, I use:

if (typeof x !== "undefined") {
    // do stuff with x
}

but I've noticed that others, eg in this question , use !== instead of != for the comparison. Which one should I be using, and why?

Note: I realize I can be using !== . The question is whether I should (and whether there will be any difference in behavior).

Answer 1

As noted in a comment by VLAZ, the typeof operator is guaranteed to return a string. If you compare the result to another string, then == and === (or != and !== ) will do the exact same thing.

Answer 2

actually, the best approach would be to check if a value is falsy , and based on MDN this is the list of falsy values:

• false The keyword false
• 0 The number zero
• 0n BigInt, when used as a boolean, follows the same rule as a Number. 0n is falsy. "", '', ``

• This is an empty string (the length of the string is zero). Strings in JavaScript can be defined with double quotes "", single quotes '', or Template literals ``.

• null null - the absence of any value

• undefined undefined - the primitive value
• NaN NaN - not a number

so based on your code what you can do is simply:

if (!x) { // check for all the falsy values.
    // do stuff with x
}

in the other hand, you ask for the difference of != and !== , well basically taking some examples you can see the difference:

0 == false   // true, because false is equivalent of 0

0 === false  // false, because both operands are of different type

2 == "2"     // true, auto type coercion, string converted into number

2 === "2"    // false, since both operands are not of same type

as mentioned by @VLAZ comment, these cases will only work if the variable x is defined, otherwise you will have the following error:

"Uncaught ReferenceError: x is not defined"

so on your case you could only use != because you will compare string vs string and you will avoid having to check if the variable was or not created.

if (typeof x != "undefined") {
    // do stuff with x
}