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This article mainly explains "how to use has () in es6". Friends who are interested might as well take a look. The method introduced in this paper is simple, fast and practical. Let's let the editor learn how to use has () in es6.

In es6, the has () method is used to intercept HasProperty operations and can also be used to hide certain properties; this method, as the in operator of the function, returns a Boolean value indicating whether there is an own or inherited attribute with the syntax of "Reflect.has (the target object of finding the property, the property to be checked)".

This tutorial operating environment: windows10 system, ECMAScript version 6. 0, Dell G3 computer.

What is the use of has () in es6

Has, as the in operator of the function, returns a Boolean value indicating whether there are own or inherited properties.

The syntax of the function has () is given below, where

Target is the target object in which you want to find the attribute.

PropertyKey is the name of the property to check.

Reflect.has (target, propertyKey)

The has () method is used to intercept HasProperty operations, that is, to determine whether an object has a property. A typical operation is the in operator.

The has () method can accept two parameters, namely the target object and the name of the property to be queried.

Use the has () method to hide some properties from being discovered by the in operator.

Var handler = {has (target, key) {if (key [0] = = "_") {return false;} return key in target;},}; var target = {_ prop: "foo", prop: "foo"}; var proxy = new Proxy (target, handler); "_ prop" in proxy; / / false

If the first character of the property name of the original object is an underscore, proxy.has () returns false so that it is not discovered by the in operator.

If the original object is not configurable or extension is disabled, the has () intercept will report an error.

Var obj = {a: 10}; Object.presentExtensions (obj); var p = new Proxy (obj, {has: function (target, prop) {return false;},}); "a" in p; / / TypeError is thrown

In the above code, the obj object forbids extension, and as a result, an error will be reported using has interception. That is, if a property is not configurable (or the target object is not extensible), the has () method must not "hide" (that is, return false) the property of the target object.

It is worth noting that the has () method intercepts the HasProperty operation, not the HasOwnProperty operation, meaning that the has () method does not determine whether an attribute is an object's own or inherited property.

In addition, although for... The in loop also uses the in operator, but has () intercepts the for. The in loop does not work.

Let stu1 = {name: "lily", score: 59}; let stu2 = {name: "lucy", score: 99}; let handler = {has (target, prop) {if (prop = "score" & & target [prop] < 60) {console.log (`${target.name} failing`); return false;} return prop in target;},}; let oproxy1 = new Proxy (stu1, handler); let oproxy2 = new Proxy (stu2, handler) "score" in oproxy1;// lily failed / / false "score" in oproxy2;// truefor (let an in oproxy1) {console.log (oproxy1 [a]);} / lily// 59for (let b in oproxy2) {console.log (oproxy2 [b]);} / / lucy// 99

In the above code, the has () interception takes effect only for the in operator, and for for... The in loop does not take effect, resulting in non-conforming properties that are not for. Excluded by the in loop.

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