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This article mainly explains the operation and application of the null merging operator. The content of the explanation is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn what is the operation and application of the null merging operator.

The null merge operator is a logical operator that returns its right Operand when the left Operand is null or undefined, otherwise it returns the left Operand.

Unlike logic or operator (| |), logic or operator returns the right Operand when the left Operand is a false value. That is, if you use | | to set default values for some variables, you may encounter unexpected behavior. For example, when it is a false value (for example,''or 0). See the example below.

Const foo = null? 'default string'; console.log (foo); / / expected output: "default string" const baz = 0? 42; console.log (baz); / / expected output: 0

Grammar

LeftExpr??rightExpr

Example

Use null merge operators

In this example, we use the null merge operator to provide default values for constants, ensuring that constants are not null or undefined.

Const nullValue = null; const emptyText = ""; / / empty string, which is a false value, Boolean ("") = false const someNumber = 42; const valA = nullValue? "default value for valA"; const valB = emptyText? "default value for valB"; const valC = someNumber? 0; console.log (valA); / / "default value for valA" console.log (valB); / / "(the empty string is a false value, but not null or undefined) console.log (valC); / / 42

Assign default values to variables

Previously, if you wanted to assign a default value to a variable, it was common practice to use logic or operator (| |):

Let foo; / / foo is never assigned any value so it is still undefined let someDummyText = foo | | 'Hellobirds'

However, because | | is a Boolean logic operator, the Operand on the left is cast to a Boolean value for evaluation. Any false values (0,', NaN, null, undefined) will not be returned. This leads to unforeseeable consequences if you use 'zero' or NaN as a valid value.

Let count = 0; let text = ""; let qty = count | | 42; let message = text | | "hi!"; console.log (qty); / / 42 instead of 0 console.log (message); / / "hi!" instead of ""

The null merge operator avoids this trap by returning the second Operand only if the first Operand is null or undefined (rather than any other false value):

Let myText =''; / / An empty string (which is also a falsy value) let notFalsyText = myText | | 'Hello world'; console.log (notFalsyText); / / Hello world let preservingFalsy = myText? 'Hi neighborhood'; console.log (preservingFalsy); / /' (as myText is neither undefined nor null)

Short circuit

Similar to the OR and AND logical operators, the right expression is not evaluated when the left expression is not null or undefined.

Function A () {console.log ('function A was called'); return undefined;} function B () {console.log ('function B was called'); return false;} function C () {console.log ('function C was called'); return "foo";} console.log (A ()? C (); / / print "function An is called", "function C is called", "foo" / / A () returns undefined, so expressions on both sides of the operator are executed console.log (B ())? C (); / / print "function B is called", "false" / / B () returns false (neither null nor undefined) / / so the right expression is not executed

Cannot be shared with AND or OR operators

Will?? It is not advisable to combine directly with AND (& &) and OR (| |) operators. (translator's note: it should be because the priority / order of operations between the null merge operator and other logical operators is undefined.) in this case, a SyntaxError is thrown.

Null | | undefined?? "foo"; / / throw SyntaxError true | | undefined?? "foo"; / / throw SyntaxError

However, there is no problem if you use parentheses to explicitly indicate the priority of the operation:

(null | | undefined)? "foo"; / / returns "foo"

The relationship with the optional chain operator (?)

The null merge operator is for the values undefined and null, as is the optional chain operator. The optional chain operator is useful when accessing objects whose properties may be undefined and null.

Let foo = {someFooProp: "hi"}; console.log (foo.someFooProp?.toUpperCase ()); / / "HI" console.log (foo.someBarProp?.toUpperCase ()); / / Thank you for your reading. The above is the content of "what is the operation and application of the null merging operator". After the study of this article, I believe that the null merging operator'? What is the operation and use of the method of this problem has a more profound understanding, the specific use of the situation also needs to be verified by practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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