对于Typescript 2.x。X你应该用以下方式(使用类型保护):

博士tl;

function isDefined<T>(value: T | undefined | null): value is T {
  return <T>value !== undefined && <T>value !== null;
}

Why?

这样，isDefined()将尊重变量的类型，下面的代码将知道这个检入帐户。

例1 -基本检查:

function getFoo(foo: string): void { 
  //
}

function getBar(bar: string| undefined) {   
  getFoo(bar); //ERROR: "bar" can be undefined
  if (isDefined(bar)) {
    getFoo(bar); // Ok now, typescript knows that "bar' is defined
  }
}

例2 -类型尊重:

function getFoo(foo: string): void { 
  //
}

function getBar(bar: number | undefined) {
  getFoo(bar); // ERROR: "number | undefined" is not assignable to "string"
  if (isDefined(bar)) {
    getFoo(bar); // ERROR: "number" is not assignable to "string", but it's ok - we know it's number
  }
}

试试这个，用!!运算符和变量。

let check;
if (!!check) {
  console.log('check is not null or not undefined');
} else {
  console.log('check is  null or  undefined');
}

它在Angular中非常有用。 检查任何变量的undefined和null。

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

对于Typescript 2.x。X你应该用以下方式(使用类型保护):

博士tl;

function isDefined<T>(value: T | undefined | null): value is T {
  return <T>value !== undefined && <T>value !== null;
}

Why?

这样，isDefined()将尊重变量的类型，下面的代码将知道这个检入帐户。

例1 -基本检查:

function getFoo(foo: string): void { 
  //
}

function getBar(bar: string| undefined) {   
  getFoo(bar); //ERROR: "bar" can be undefined
  if (isDefined(bar)) {
    getFoo(bar); // Ok now, typescript knows that "bar' is defined
  }
}

例2 -类型尊重:

function getFoo(foo: string): void { 
  //
}

function getBar(bar: number | undefined) {
  getFoo(bar); // ERROR: "number | undefined" is not assignable to "string"
  if (isDefined(bar)) {
    getFoo(bar); // ERROR: "number" is not assignable to "string", but it's ok - we know it's number
  }
}

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);
  });
});

我有这个问题，一些答案工作只是很好的JS，但不是TS这里的原因。

//JS
let couldBeNullOrUndefined;
if(couldBeNullOrUndefined == null) {
  console.log('null OR undefined', couldBeNullOrUndefined);
} else {
  console.log('Has some value', couldBeNullOrUndefined);
}

这很好，因为JS没有类型

//TS
let couldBeNullOrUndefined?: string | null; // THIS NEEDS TO BE TYPED AS undefined || null || Type(string)

if(couldBeNullOrUndefined === null) { // TS should always use strict-check
  console.log('null OR undefined', couldBeNullOrUndefined);
} else {
  console.log('Has some value', couldBeNullOrUndefined);
}

在TS中，如果变量未定义为null，当您试图检查该null时，tslint |编译器将报错。

//tslint.json
...
"triple-equals":[true],
...

 let couldBeNullOrUndefined?: string; // to fix it add | null

 Types of property 'couldBeNullOrUndefined' are incompatible.
      Type 'string | null' is not assignable to type 'string | undefined'.
        Type 'null' is not assignable to type 'string | undefined'.