如果你正在使用TypeScript，让编译器检查空值和未定义值(或可能存在的)是一个更好的方法，而不是在运行时检查它们。(如果您确实想在运行时检查，那么正如许多答案所表明的那样，只需使用value == null)。

使用编译选项strictNullChecks告诉编译器阻塞可能的空值或未定义值。如果你设置了这个选项，然后有一个你想要允许null和undefined的情况，你可以定义类型为type | null | undefined。

如果你使用本地存储，小心，你可能会以字符串undefined而不是值undefined结束:

localStorage.setItem('mykey',JSON.stringify(undefined));
localStorage.getItem('mykey') === "undefined"
true

人们可能会发现这个很有用:https://github.com/angular/components/blob/master/src/cdk/coercion/boolean-property.spec.ts

/**
 * @license
 * Copyright Google LLC All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */

/** Coerces a data-bound value (typically a string) to a boolean. */
export function coerceBooleanProperty(value: any): boolean {
  return value != null && `${value}` !== 'false';
}

import {coerceBooleanProperty} from './boolean-property';

describe('coerceBooleanProperty', () => {

  it('should coerce undefined to false', () => {
    expect(coerceBooleanProperty(undefined)).toBe(false);
  });

  it('should coerce null to false', () => {
    expect(coerceBooleanProperty(null)).toBe(false);
  });

  it('should coerce the empty string to true', () => {
    expect(coerceBooleanProperty('')).toBe(true);
  });

  it('should coerce zero to true', () => {
    expect(coerceBooleanProperty(0)).toBe(true);
  });

  it('should coerce the string "false" to false', () => {
    expect(coerceBooleanProperty('false')).toBe(false);
  });

  it('should coerce the boolean false to false', () => {
    expect(coerceBooleanProperty(false)).toBe(false);
  });

  it('should coerce the boolean true to true', () => {
    expect(coerceBooleanProperty(true)).toBe(true);
  });

  it('should coerce the string "true" to true', () => {
    expect(coerceBooleanProperty('true')).toBe(true);
  });

  it('should coerce an arbitrary string to true', () => {
    expect(coerceBooleanProperty('pink')).toBe(true);
  });

  it('should coerce an object to true', () => {
    expect(coerceBooleanProperty({})).toBe(true);
  });

  it('should coerce an array to true', () => {
    expect(coerceBooleanProperty([])).toBe(true);
  });
});

我在typescript操场上做了不同的测试:

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);

给:

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

so:

检查(a == null)是否正确，以知道a是否为空或未定义 检查(a != null)是否正确，以知道是否定义了a 检查(a)是否错误，以知道a是否被定义

最简单的方法是使用:

import {isNullOrUndefined} from 'util';

比:

如果isNullOrUndefined (foo (!)

更新(2020年9月4日)

你现在可以使用??运算符来验证空值和未定义的“值”并设置默认值。例如:

const foo = null;
const bar = foo ?? 'exampleValue';
console.log(bar); // This will print 'exampleValue' due to the value condition of the foo constant, in this case, a null value

作为一种详细的方式，如果你想比较空值和未定义值，使用下面的示例代码作为参考:

const incomingValue : string = undefined;
const somethingToCompare : string = incomingValue; // If the line above is not declared, TypeScript will return an excepion

if (somethingToCompare == (undefined || null)) {
  console.log(`Incoming value is: ${somethingToCompare}`);
}

如果incomingValue没有声明，TypeScript应该返回一个异常。如果声明了但没有定义，console.log()将返回“Incoming value is: undefined”。注意，我们没有使用严格的等于运算符。

“正确”的方法(检查其他答案的细节)，如果incomingValue不是一个布尔类型，只是评估它的值是否为真，这将根据常量/变量类型进行评估。真正的字符串必须使用= "赋值显式地定义为字符串。否则，它将被赋值为false。让我们使用相同的上下文来检查这个情况:

const incomingValue : string = undefined;
const somethingToCompare0 : string = 'Trumpet';
const somethingToCompare1 : string = incomingValue;

if (somethingToCompare0) {
  console.log(`somethingToCompare0 is: ${somethingToCompare0}`); // Will return "somethingToCompare0 is: Trumpet"
}

// Now, we will evaluate the second constant
if (somethingToCompare1) {
  console.log(`somethingToCompare1 is: ${somethingToCompare1}`); // Launched if incomingValue is defined
} else {
  console.log(`somethingToCompare1 is: ${somethingToCompare1}`); // Launched if incomingValue is undefined. Will return "somethingToCompare1 is: undefined"
}

晚加入这个线程，但我发现这个JavaScript黑客在检查一个值是否未定义非常方便

 if(typeof(something) === 'undefined'){
   // Yes this is undefined
 }