What does javascript:void mean?

javascript:void is a usage format of the void operator in Javascript; the void operator specifies to calculate an expression but does not return a value; developers can use void to prevent a tag from jumping when clicked , you can also use void0 to take undefined.

The operating environment of this article: windows7 system, javascript version 1.8.5, Dell G3 computer.

void in Javascript is an operator that specifies to evaluate an expression but not return a value. You can use void to prevent the a tag from jumping when clicked. You can also use void0 to take undefined.

void Operator Usage

javascript:void (expression)
javascript:void expression

expression is a Javascript standard expression to be evaluated. The parentheses outside the expression are optional, but are a good practice to write.

You can use the void operator to specify a hyperlink. The expression will be evaluated but nothing will be loaded into the current document. The above code creates a hyperlink and nothing happens to the user later. Nothing happens on Javascript when the user links.

Description in the ECMAScript 262 specification

The void Operator
The production UnaryExpression : void UnaryExpression is evaluated as follows:
Let expr be the result of evaluating UnaryExpression.
Call GetValue(expr).
Return undefined.
NOTE: GetValue must be called even though its value is not used because it may have 
observable side-effects.

It can be understood from the specification that no matter what the expression after void is, the void operator will return undefined

[Recommended learning: js basic tutorial]

Why use void

undefined is not a reserved word or keyword in Javascript

function isUndefined(param) {
    let undefined = 'hello world';    
    return undefined === param;
}

We can define a variable called undefined in the context of a function. At this time, if we want to access the real undefined, we can only access undefined through the global scope.

window.undefined;
GLOBAL.undefined;

Unfortunately, window and GLOBAL can still be defined in a function context, so taking undefined from them is not a safe and reliable method

function test() {    let 
        undefined = 'hello world',        
        window = { undefined: 'joke' },
        f = {}
    ;    
        console.log(undefined);    
        console.log(window.undefined);    
        console.log(f.a === undefined);    
        console.log(f.a === void 0);
}

Judge undefined

function isUndefined(param) { 
   return param === void 0;
}
另外一种方式
function getUndefined() {
    return;
};
    function isUndefined(param) {    
    return param === getUndefined();
}

This method is feasible because a function that does not specify a return value will By default, undefined is returned. The disadvantage is that in order to determine undefined, a function must be declared, which will cause a loss in performance.

Priority

void Priority is second only to. [] ()

Summary:

It is safer to use void 0 to get undefined than to use literal undefined. Void 0 should be used first.

Fill in the href of to ensure that no page jump occurs when clicked. Avoid adding click events to the a tag and use return false to organize the default behavior.

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