A closer look at null in JavaScript

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JavaScript has 2 types: basic type (string, booleans number, symbol) and objects.

Objects are complex data structures. The simplest objects in JS are ordinary objects: a set of keys and associated values:

let myObject = {
  name: '前端小智'
}

But in some cases, objects cannot be created. In this case, JS provides a special value null — indicating that the object is missing.

let myObject = null

In this article, we will learn everything about null in JavaScript: what it means, how to detect it, the difference between null and undefined And why using null makes code maintenance difficult.

1. The concept of null

The JS specification explains information about null:

The value null refers to an object whose value is not set. It is one of the basic types of JS and is considered falsy in Boolean operations.

For example, the function greetObject() creates an object, but it can also return null when the object cannot be created:

function greetObject(who) {
  if (!who) {
    return null;
  }
  return { message: `Hello, ${who}!` };
}

greetObject('Eric'); // => { message: 'Hello, Eric!' }
greetObject();       // => null

However, if When the function greetObject() is called with parameters, the function returns null. Returning null is reasonable because the who parameter has no value.

2. How to check for null

A good way to check for a null value is to use the strict equality operator:

const missingObject = null;
const existingObject = { message: 'Hello!' };

missingObject  === null; // => true
existingObject === null; // => false

missingObject === null results in true because the missingObject variable contains a null value.

If the variable contains a non-null value (such as an object), the expression existObject === null evaluates to false.

2.1 null is a virtual value

null and false, 0, '', undefined, NaN are all imaginary values. If a false value is encountered in a conditional statement, JS will force the false value to false.

Boolean(null); // => false

if (null) {
  console.log('null is truthy')
} else {
  console.log('null is falsy')
}

2.2 typeof null

typeof valueoperator determines the type of value. For example, typeof 15 is 'number', and the calculation result of typeof {prop: 'Value'} is 'object'.

Interestingly, what is the result of type null

typeof null; // => 'object'

Why is 'object', typoef null is object is a bug in early JS implementations.

To detect null values, use the typeof operator. As mentioned before, use the strict equality operator myVar === null.

If we want to use the typeof operator to check if a variable is an object, we also need to exclude the null value:

function isObject(object) {
  return typeof object === 'object' && object !== null;
}

isObject({ prop: 'Value' }); // => true
isObject(15);                // => false
isObject(null);              // => false

3. The trap of null

null often appears unexpectedly when we think the variable is an object. Then, if you extract the property from null, JS will throw an error.

Use the greetObject() function again and try to access the message property from the returned object:

let who = '';

greetObject(who).message; 
// throws "TypeError: greetObject() is null"

because who The variable is an empty string, so the function returns null. When accessing the message property from null, a TypeError error is raised.

Can be handled by using optional chaining with null mergingnull:

let who = ''

greetObject(who)?.message ?? 'Hello, Stranger!'
// => 'Hello, Stranger!'

4. Alternatives to null

When the object cannot be constructed, our usual approach is to return null, but this approach has shortcomings. When null appears in the execution stack, a check must be performed.

Try to avoid returning null:

• return the default object instead of null
• throwing an error Instead of returning null

, return to the greetObject() function that originally returned the greeting object. When parameters are missing, you can return a default object instead of returning null:

function greetObject(who) {
  if (!who) {
    who = 'Stranger';
  }
  return { message: `Hello, ${who}!` };
}

greetObject('Eric'); // => { message: 'Hello, Eric!' }
greetObject();       // => { message: 'Hello, Stranger!' }

or throw an error:

function greetObject(who) {
  if (!who) {
    throw new Error('"who" argument is missing');
  }
  return { message: `Hello, ${who}!` };
}

greetObject('Eric'); // => { message: 'Hello, Eric!' }
greetObject();       // => throws an error

These two approaches can avoid using null.

5. null vs undefined

undefined is the value of an uninitialized variable or object property, and undefined is the value of an uninitialized variable or object property. The main difference between

let myVariable;

myVariable; // => undefined

null and undefined is that null represents a missing object while undefined represents Uninitialized state.

Strict equality operator operator===Distinguish between null and undefined:

null === undefined // => false

And double equality operator== is considered to be equal to null and undefined

null == undefined // => true

我使用双等相等运算符检查变量是否为null 或undefined:

function isEmpty(value) {
  return value == null;
}

isEmpty(42);                // => false
isEmpty({ prop: 'Value' }); // => false
isEmpty(null);              // => true
isEmpty(undefined);         // => true

6. 总结

null是JavaScript中的一个特殊值，表示丢失的对象，严格相等运算符确定变量是否为空:variable === null。

typoef运算符对于确定变量的类型（number, string, boolean）很有用。 但是，如果为null，则typeof会产生误导：typeof null的值为'object'。

null和undefined在某种程度上是等价的，但null表示缺少对象，而undefined未初始化状态。

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