JavaScript-tillbehör


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JavaScript-tillbehör (Getters och Setters)

ECMAScript 5 (ES5 2009) introducerade Getter och Setters.

Getters och seters låter dig definiera objekttillbehör (Computed Egenskaper).


JavaScript Getter (Get-sökordet)

Det här exemplet använder en lang-egenskap för att värdet för egenskapen language.

Exempel

// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "en",
  get lang() {
    return this.language;
    }
};

// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;

Prova själv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Getters and setters allow you to get and set object properties via methods.</p>

<p>This example uses a lang property to get the value of the language property:</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "en",
  get lang() {
    return this.language;
  }
};

// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;
</script>

</body>
</html>

JavaScript Setter (det uppsatta nyckelordet)

Det här exemplet använder en lang-egenskap för att ställa värdet för egenskapen language.

Exempel

const person = {
  firstName: "John",
  lastName: "Doe",
  language: "",
    set lang(lang) {
    this.language = lang;
    }
};

// Set an object 
property using a setter:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;

Prova själv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Getters and setters allow you to get and set properties via methods.</p>

<p>This example uses a lang property to set the value of the language property.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "NO",
  set lang(value) {
    this.language = value;
  }
};

// Set a property using set:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;
</script>

</body>
</html>


JavaScript-funktion eller Getter?

Vad är skillnaden mellan dessa två exempel?

Exempel 1

const person = {
  firstName: "John",
  lastName: "Doe",
  fullName: function() {
    return this.firstName + " " + 
this.lastName;
  }
};
// Display data from the object using a method:
  document.getElementById("demo").innerHTML = person.fullName();

Prova själv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object Methods</h2>

<p>Object data can be accessed using property stored as a function.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }
};

// Display data from the object using a method:
document.getElementById("demo").innerHTML = person.fullName();
</script>

</body>
</html>

Exempel 2

const person = {
  firstName: "John",
  lastName: "Doe",
  get fullName() {
    return this.firstName + " " + 
this.lastName;
  }
};
// Display data from the object using a getter:
  document.getElementById("demo").innerHTML = person.fullName;

Prova själv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object Methods</h2>

<p>Object data can be accessed using a getter.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  get fullName() {
    return this.firstName + " " + this.lastName;
  }
};
// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.fullName;
</script>

</body>
</html>

Exempel 1 få tillgång till fullName som en funktion: person.fullName().

Exempel 2 få tillgång till fullName som en egenskap: person.fullName.

Det andra exemplet ger en enklare syntax.


Datakvalitet

JavaScript kan säkra bättre datakvalitet när du använder getters och setters.

Genom att använda egenskapen lang i det här exemplet returneras värdet av egenskapen language med versaler:

Exempel

// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "en",
  get lang() {
    return this.language.toUpperCase();
    }
};

// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;

Prova själv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Getters and setters allow you to get and set properties via methods.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "en",
  get lang() {
    return this.language.toUpperCase();
  }
};
// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;
</script>

</body>
</html>

Genom att använda egenskapen lang lagras i det här exemplet en versal värde i egenskapen language:

Exempel

const person = {
  firstName: "John",
  lastName: "Doe",
  language: "",
  set lang(lang) {
      this.language = lang.toUpperCase();
    }
};

// Set an object 
property using a setter:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;

Prova själv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Getters and setters allow you to get and set properties via methods.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "",
  set lang(lang) {
    this.language = lang.toUpperCase();
  }
};

// Set a property using set:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;
</script>

</body>
</html>

Varför använda getters och setters?

  • Det ger enklare syntax

  • Det tillåter lika syntax för egenskaper och metoder

  • Det kan säkra bättre datakvalitet

  • Det är användbart för att göra saker bakom kulisserna


Object.defineProperty()

Metoden Object.defineProperty() kan också användas för att lägga till Getters och Setters:

Ett motexempel

// Define object
const obj = {counter : 0};

// Define setters and getters
Object.defineProperty(obj, "reset", {
  get : function () {this.counter = 0;}
});
Object.defineProperty(obj, "increment", {
  get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
  get : function () {this.counter--;}
});
Object.defineProperty(obj, "add", {
  set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
  set : function (value) {this.counter -= value;}
});

// Play with the counter:
obj.reset;
obj.add = 5;
obj.subtract = 1;
obj.increment;
obj.decrement;

Prova själv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Perfect for creating counters:</p>

<p id="demo"></p>

<script>
// Define an object
const obj = {counter : 0};

// Define Setters and Getters
Object.defineProperty(obj, "reset", {
  get : function () {this.counter = 0;}
});
Object.defineProperty(obj, "increment", {
  get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
  get : function () {this.counter--;}
});
Object.defineProperty(obj, "add", {
  set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
  set : function (value) {this.counter -= value;}
});

// Play with counter:
obj.reset;
obj.add = 5;
obj.subtract = 1;
obj.increment;
obj.decrement;
document.getElementById("demo").innerHTML = obj.counter;
</script>

</body>
</html>