在 javascript 中使用 != null 和 != undefined 之间有什么功能区别吗？

Question

In javascript, is there any functional difference between using != null and != undefined ?

Is there a value you can assign to myVar which will cause these two lines of code to evaluate to different results?

console.log(myVar != undefined)
console.log(myVar != null)

And if you know anything about the performance of these two operations, I'd love to know about that too.

Answer 1

There is no functional difference. x != undefined and x != null both only evaluate to false when x is null or undefined . They both evaluate to true for all other values of x .

There is also no performance difference.

Answer 2

There is no difference as you can see in below table for JS == testing (focus on null/undefined row/column) (src: here ). So myVar!=null is true only if myVar value is not null and not undefined (same with myVar != undefined )

It looks like both has similar performance (I made test on Mac OS X 10.13.4 HighSierra: Chrome 71.0.3578, Firefox 65.0.0 and Safari 11.1.0 - you can run test in your browser here )

let myVar1=null;
let myVar2=undefined;

Answer 3

The == and != operators make "type conversion" to compare just the value itself. Then no, there's no difference using "undefinied" or "null" is this case, both represents "empty".

But if you use === and !== instead, it checks the type and the value, without any type casting. The results for the two lines would be different.

myVar = null;
console.log(myVar !== undefined) //true
console.log(myVar !== null) //false

Answer 4

Don't confuse undefined and null as they are not the same thing.

null:

The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values.

undefined:

A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned.

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If the variable consists of a value that is neither null nor undefined , then no there is no difference in your condition.

 const value = 3; console.log(value !== undefined) //true console.log(value !== null) //true

However, a better way of testing if the variable is null or undefined is to use ! negation as the value null or undefined will be resolved to true.

 const undefinedValue = undefined; const nullValue = null; console.log(!undefinedValue); console.log(!nullValue);

Here are some examples of it.

 var someVariable = undefined; console.log(someVariable !== undefined, "; undefined !== undefined"); console.log(someVariable !== null, "; undefined !== null"); var someVariable = null; console.log(someVariable !== undefined, "; null !== undefined"); console.log(someVariable !== null, "; null !== null"); var someVariable = undefined; console.log(!someVariable); var someVariable = null; console.log(!someVariable);