官术网_书友最值得收藏!

How 'this' works within JavaScript

Within JavaScript, this has varying contexts that range from the global window context to eval, newable, and function contexts. As the default context for this relates to the global scope, this is our window object:

/**
* Outputting the value of this to the console in the global context returns the Window object
*/
console.log(this);

/**
* When referencing global Window objects, we don't need to refer to them with this, but if we do, we get the same behavior
*/
alert('Alert one');
this.alert('Alert two');

The context of this changes depending on where we are in scope. This means, that if we had a Student object with particular values, such as firstName, lastName, grades, and so on, the context of this would be related to the object itself:

/**
* The context of this changes when we enter another lexical scope, take our Student object example:
*/
const Student = {
firstName: 'Paul',
lastName: 'Halliday',
grades: [50, 95, 70, 65, 35],
getFullName() {
return `${this.firstName} ${this.lastName}`
},
getGrades() {
return this.grades.reduce((accumulator, grade) => accumulator + grade);
},
toString() {
return `Student ${this.getFullName()} scored ${this.getGrades()}/500`;
}
}

When we run the preceding code with console.log(Student.toString()), we get this: Student Paul Halliday scored 315/500 as the context of this is now scoped to the object itself rather than the global window scope.

If we wanted to display this in the document we could do it like so:

let res = document.createTextNode(Student.toString());
let heading = document.createElement('h1');
heading.appendChild(res);
document.body.appendChild(heading);

Notice that, with the preceding code, once again we don't have to use this as it isn't needed with the global context.

Now that we have an understanding of how this works at a basic level, we can investigate how Vue proxies this inside of our instances to make interacting with the various properties much easier.

主站蜘蛛池模板: 屏边| 凤翔县| 明水县| 巴东县| 南乐县| 淳安县| 丹东市| 吉木萨尔县| 祥云县| 清镇市| 泸州市| 安图县| 板桥市| 孝感市| 白沙| 内黄县| 东乌珠穆沁旗| 阜新| 张家川| 汶上县| 镇原县| 万年县| 宾川县| 刚察县| 通化市| 开平市| 黔西| 盘山县| 万安县| 嘉祥县| 美姑县| 历史| 东乌珠穆沁旗| 永平县| 顺平县| 灵丘县| 专栏| 新闻| 平罗县| 宜州市| 增城市|