有没有办法检查`null`和`undefined`？

本文翻译自：Is there a way to check for both `null` and `undefined`?

Since TypeScript is strongly-typed, simply using if () {} to check null and undefined doesn't sound right. 由于TypeScript是强类型的，因此仅使用if () {}来检查null和undefined听起来并不正确。

Does TypeScript has dedicated function or syntax sugar for this? TypeScript是否为此具有专用功能或语法糖？

---

#1楼

参考：https://stackoom.com/question/1xZxA/有没有办法检查-null-和-undefined

---

#2楼

Does TypeScript has dedicated function or syntax sugar for this TypeScript是否为此具有专用功能或语法糖

No. I just do something == null same as JavaScript. 不。我只是做something == null与JavaScript相同。

---

#3楼

I always write it like this: 我总是这样写：

var foo:string;

if(!foo){
   foo="something";    
}

This will work fine and I think it's very readable. 这可以正常工作，我认为它非常可读。

---

#4楼

Using a juggling-check, you can test both null and undefined in one hit: 使用杂项检查，您可以在一次undefined中同时测试null和undefined ：

if (x == null) {

If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables: 如果使用严格检查，则仅对于设置为null值才为true，而对于未定义的变量则为true：

if (x === null) {

You can try this with various values using this example: 您可以使用以下示例尝试各种值：

var a: number;
var b: number = null;

function check(x, name) {
    if (x == null) {
        console.log(name + ' == null');
    }

    if (x === null) {
        console.log(name + ' === null');
    }

    if (typeof x === 'undefined') {
        console.log(name + ' is undefined');
    }
}

check(a, 'a');
check(b, 'b');

Output 输出量

"a == null" “一个==空”

"a is undefined" “一个未定义”

"b == null" “ b == null”

"b === null" “ b === null”

---

#5楼

I did different tests on the typescript playground: 我在打字稿游乐场进行了不同的测试：

http://www.typescriptlang.org/play/ http://www.typescriptlang.org/play/

let a;
let b = null;
let c = "";
var output = "";

if (a == null) output += "a is null or undefined\n";
if (b == null) output += "b is null or undefined\n";
if (c == null) output += "c is null or undefined\n";
if (a != null) output += "a is defined\n";
if (b != null) output += "b is defined\n";
if (c != null) output += "c is defined\n";
if (a) output += "a is defined (2nd method)\n";
if (b) output += "b is defined (2nd method)\n";
if (c) output += "c is defined (2nd method)\n";

console.log(output);

gives: 给出：

a is null or undefined
b is null or undefined
c is defined

so: 所以：

• checking if (a == null) is right to know if a is null or undefined 检查（a == null）是否正确以知道a为null还是未定义
• checking if (a != null) is right to know if a is defined 检查（a！= null）是否正确以了解是否已定义a
• checking if (a) is wrong to know if a is defined 检查（a）是否错误以知道是否已定义a

---

#6楼

if(data){}

it's mean !data 意思是！数据

• null 空值
• undefined 未定义
• false 假
• .... ....